Having over 80 addons makes me really /hate/ having to do testing with disabling and stuff.
I will if you really need me to, though.
I've noticed it seems to happen if you attempt to post multiple items. 1 stack doesn't tend to cause the bug, but something that's more than one stack does.
I attempt to post, and get an error, I don't have the money required to post it.
If I wait, it'll say "error attempting to post [copper bar]x20" (and give the not enough money error again)
And if I wait some more
"error attempting to post [copper bar]x9" (and give the not enough money error again)
If, however, I do NOT wait for those messages and proceed to attempt to post other auctions, or leave the AH and go vendor, or in general be an impatient person, that's when the error pops up.