Reproducing is easy:

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$ cat C.java; for i in 1.4 1.5 1.6; do /space/jdk$i/bin/javadoc -J-showversion C.java; done
/** <a> */
public class C {}
java version "1.4.2_08"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_08-b03)
Java HotSpot(TM) Client VM (build 1.4.2_08-b03, mixed mode)

Loading source file C.java...
Constructing Javadoc information...
Standard Doclet version 1.4.2_08
Generating constant-values.html...
Building tree for all the packages and classes...
Building index for all the packages and classes...
Generating overview-tree.html...
Generating index-all.html...
Generating deprecated-list.html...
Building index for all classes...
Generating allclasses-frame.html...
Generating allclasses-noframe.html...
Generating index.html...
Generating packages.html...
Generating C.html...
Generating package-list...
Generating help-doc.html...
Generating stylesheet.css...
java version "1.5.0_04"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.5.0_04-b05)
Java HotSpot(TM) Client VM (build 1.5.0_04-b05, mixed mode, sharing)

Loading source file C.java...
Constructing Javadoc information...
Standard Doclet version 1.5.0_04
Building tree for all the packages and classes...
Generating C.html...
Generating package-frame.html...
Generating package-summary.html...
Generating package-tree.html...
Generating constant-values.html...
Building index for all the packages and classes...
Generating overview-tree.html...
Generating index-all.html...
java.util.MissingResourceException: Can't find resource for bundle com.sun.tools.doclets.formats.html.resources.standard, key doclet.malformed_html_link_tag
	at java.util.ResourceBundle.getObject(ResourceBundle.java:326)
	at java.util.ResourceBundle.getString(ResourceBundle.java:286)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.getText(MessageRetriever.java:114)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.getText(MessageRetriever.java:92)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.getText(MessageRetriever.java:81)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.warning(MessageRetriever.java:290)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.redirectRelativeLinks(HtmlDocletWriter.java:1526)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.commentTagsToString(HtmlDocletWriter.java:1438)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.printCommentTags(HtmlDocletWriter.java:1397)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.printSummaryComment(HtmlDocletWriter.java:1370)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.printSummaryComment(HtmlDocletWriter.java:1366)
	at com.sun.tools.doclets.formats.html.AbstractIndexWriter.printComment(AbstractIndexWriter.java:192)
	at com.sun.tools.doclets.formats.html.AbstractIndexWriter.printDescription(AbstractIndexWriter.java:126)
	at com.sun.tools.doclets.formats.html.AbstractIndexWriter.generateContents(AbstractIndexWriter.java:91)
	at com.sun.tools.doclets.formats.html.SingleIndexWriter.generateIndexFile(SingleIndexWriter.java:76)
	at com.sun.tools.doclets.formats.html.SingleIndexWriter.generate(SingleIndexWriter.java:52)
	at com.sun.tools.doclets.formats.html.HtmlDoclet.generateOtherFiles(HtmlDoclet.java:103)
	at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:122)
	at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractDoclet.java:64)
	at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:42)
	at com.sun.tools.doclets.standard.Standard.start(Standard.java:23)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:269)
	at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:143)
	at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:340)
	at com.sun.tools.javadoc.Start.begin(Start.java:128)
	at com.sun.tools.javadoc.Main.execute(Main.java:41)
	at com.sun.tools.javadoc.Main.main(Main.java:31)
java version "1.6.0-ea"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.6.0-ea-b45)
Java HotSpot(TM) Client VM (build 1.6.0-ea-b45, mixed mode, sharing)

Loading source file C.java...
Constructing Javadoc information...
Standard Doclet version 1.6.0-ea
Building tree for all the packages and classes...
Generating C.html...
Generating package-frame.html...
Generating package-summary.html...
Generating package-tree.html...
Generating constant-values.html...
Building index for all the packages and classes...
Generating overview-tree.html...
Generating index-all.html...
java.util.MissingResourceException: Can't find resource for bundle com.sun.tools.doclets.formats.html.resources.standard, key doclet.malformed_html_link_tag
	at java.util.ResourceBundle.getObject(ResourceBundle.java:369)
	at java.util.ResourceBundle.getString(ResourceBundle.java:329)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.getText(MessageRetriever.java:114)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.getText(MessageRetriever.java:92)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.getText(MessageRetriever.java:81)
	at com.sun.tools.doclets.internal.toolkit.util.MessageRetriever.warning(MessageRetriever.java:290)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.redirectRelativeLinks(HtmlDocletWriter.java:1526)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.commentTagsToString(HtmlDocletWriter.java:1438)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.printCommentTags(HtmlDocletWriter.java:1397)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.printSummaryComment(HtmlDocletWriter.java:1370)
	at com.sun.tools.doclets.formats.html.HtmlDocletWriter.printSummaryComment(HtmlDocletWriter.java:1366)
	at com.sun.tools.doclets.formats.html.AbstractIndexWriter.printComment(AbstractIndexWriter.java:192)
	at com.sun.tools.doclets.formats.html.AbstractIndexWriter.printDescription(AbstractIndexWriter.java:126)
	at com.sun.tools.doclets.formats.html.AbstractIndexWriter.generateContents(AbstractIndexWriter.java:91)
	at com.sun.tools.doclets.formats.html.SingleIndexWriter.generateIndexFile(SingleIndexWriter.java:76)
	at com.sun.tools.doclets.formats.html.SingleIndexWriter.generate(SingleIndexWriter.java:52)
	at com.sun.tools.doclets.formats.html.HtmlDoclet.generateOtherFiles(HtmlDoclet.java:103)
	at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.startGeneration(AbstractDoclet.java:122)
	at com.sun.tools.doclets.internal.toolkit.AbstractDoclet.start(AbstractDoclet.java:64)
	at com.sun.tools.doclets.formats.html.HtmlDoclet.start(HtmlDoclet.java:42)
	at com.sun.tools.doclets.standard.Standard.start(Standard.java:23)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:585)
	at com.sun.tools.javadoc.DocletInvoker.invoke(DocletInvoker.java:215)
	at com.sun.tools.javadoc.DocletInvoker.start(DocletInvoker.java:91)
	at com.sun.tools.javadoc.Start.parseAndExecute(Start.java:340)
	at com.sun.tools.javadoc.Start.begin(Start.java:128)
	at com.sun.tools.javadoc.Main.execute(Main.java:41)
	at com.sun.tools.javadoc.Main.main(Main.java:31)
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As you can see, JDK 1.5 (and 1.6) try to issue a warning about the Javadoc comment (which is in fact using invalid HTML) but that message resource does not seem to have been included in the tool.