We have a legacy site running EE 1.6.9, and the Forum module 2.1.0. (Upgrading is not an option due to changes needed in dozens of templates.)
We have just noticed that when running the forum through EE templates, we are no longer getting the Post Submission validation error messages. We do get them when running just through the forum templates.
We've also tried changing the theme from our custom one, to all of the other default themes, and see the same issue.
Does anyone else remember having this issue, or know of a fix? I can't even seem to find the EE 1.x bug tracker any longer.