John Kerry
Kerry said that he had intended the remark as a jab at President Bush, and described the remarks as a "botched joke", having inadvertently left out the key word "us" (which would have been, "If you don't, you get us stuck in Iraq"), as well as leaving the phrase "just ask President Bush" off of the end of the sentence. In Kerry's prepared remarks, which he released during the ensuing media frenzy, the corresponding line was "... you end up getting us stuck in a war in Iraq. Just ask President Bush." He also said that from the context of the speech which, prior to the "stuck in Iraq" line, made several specific references to Bush and elements of his biography, that Kerry was referring to President Bush and not American troops in general.

Q: What mistake did Kerry make in the joke?
inadvertently left out the key word "us"