forum code is fucked up. it doesn't help that if you are in talk back, you can't go back and edit to fix the random changes to [code] that the forum will make without your input or knowledge until you hit the send button.
sometime it will do it if you hit the backspace.
lots of time it will format quoted text from another site and add all sorts of stuff, like font, color, size, and other unintended code that shouldn't have carried over, and might not have even appeared to have been present to begin with. It's frustrating, but that's why you gotta click the "Red A" before pasting anything and preview before posting.