Element insertion in CHT with G1 is typically  structured as follows:

_table.insert(..., &grow_hint);
[...]
if (grow_hint) {
  _table.grow();
}

The problem is that in the time between insertion and growing additional threads can queue up to grow the table.

E.g.
Thread A: insert(... , grow_hint = true);
Thread B: insert(... , grow_hint = true);
Thread A: grow()
Thread B: grow()

There is some simplistic mechanism to cancel table growing in CHT's grow() call, but it does not seem sufficient.

This is particularly apparent after use of a CHT with the same structure for code roots in JDK-8315503, but can also be seen with the card set to a lesser degree.