The maximum length of the field for post subjects is 100 chars. It is also limited by maxlengh of the text input.
But when you have a post with a subject 100 chars long and someone replies,
the subject becomes longer than 100 chars because of the "Re:".
That fails the insert.
Right now, it seems that the application does not validate the input, so with PostgreSQL, I switched those columns to text (from varchar).
In PostgreSQL, there is no downside to using text (no performance loss).
I don't know about other DBs.