Lets say we have a custom security manager that uses string concatenation (+) inside the method checkPermission(Permission).
When we call the method that does permission checking, it throws a java.lang.BootstrapMethodError.

Minimized test: 
-----------------------------------------

    SecurityManager sm = new SecurityManager() {

            @Override
            public void checkPermission(Permission perm) {
                String abc = "abc";
                String full = abc + "def";
            }
        };
        System.setSecurityManager(sm);
        ClassLoader cl = new ClassLoader() {};
    }
-----------------------------------------

Interesting that the spec for j.l.String says the following regarding concatenation: 
"The Java language provides special support for the string concatenation operator ( + ), ... String concatenation is implemented through the StringBuilder(or StringBuffer) class and its append method."
However, if we will replace {abc + "def"} with {new StringBuffer().append(abc).append("def").toString()}, the exception won't be thrown.

Reproduced with jdk9b104 and later.