It's an oddity that happens with long thread titles and is caused by one of the add-ons I've got install - but I don't want to remove the add-on because it's useful ...
If you get this notice when trying to use the report button either post here to let me know or send me a PM and I'll sort it (by contracting the thread title
).
I have reported the problem to the add-on developer but they haven't been back to XF in quite a while so I'm not confident it will be resolved directly, although the features of the add-on may eventually be built-in to Xenforo which should "fix" it.
Cheers,
Shaun