Hmmm. Testing this, so it doesn't like selective?
Apparently its inconsistent about it, since this posted.
Selective enforcement seems to be part of why these errors don't always show up, and some people experience them more often.
It's worth noting that VBulletin is constantly updating to protect against SQL injection attacks, so it's quite possible that this is security in action.
All I really know about it is I discussed the issue with a poster who's had the issue before, and they mentioned that "sel/ec/tive" was a word they'd found to trigger it (also "oh/io" and "rac/ial").
Sure enough I checked the text of a post I tried to post every time the board would let me back in and the word "sel/ec/tive" was right there. This current cycle I replaced that word with another and posted the same text and have had no problem.
Not saying you're wrong, Silvermooncat; if you tested, that's data. Its just interesting that it apparently also is connected with the user.
for what it's worth, I first encountered this around April 22nd of this year
so, one factor in the SNR bug is that it appears to be caused by provoking the front page bug twice in succession