I actually do remember this happening once before that I noticed. It's just weird that it only seems to happen on specific topics.
(My luck, it just had to be on the 1 topic I actively post on, lol)
And when I say white screen... I get body tags and that's it and get told the page has no source code.
Edit: Dunno if it matters but when I posted that I got caught by Firn's test post and it worked normally telling me there was a new post. It then proceeded to white screen when I hit post again to actually put the message through.