Some of the sillier things I've been told or have read over the years:
Shooting an animal w/ an 'assault weapon' destroys it. (no comment when real specs on them were presented)
The NRA put out an advertisement supporting AP ammo because "Some animals are armored". (oh a friend told him about it)
You can make a semi-auto into a machine gun by filing down the FIRING pin. (complete ignorance of firearm operation)
So why bother? Unless you're actually debating publicly some public figure what good does it do?
Well it's not always a complete waste of time.
Depending on the forum, odds are good there are 'fence-sitters' there as well. Countering the misinformation and emotional appeals presented can help either keep them neutral or sway them towards positive. One of the best ways to do this is to stay calm and just keep countering them. In my experience they will invariable resort to ad hominems (usually involving the NRA), character attacks or the ever popular comments on genitalia size.
You can also learn from them. Really, you can. I've gotten some really good sources from anti's who cited a study or some historical document but either partially quoted it, took it out of context or just assumed what it was about w/o reading the whole thing, sometimes getting it completely backwards.
Of course sometimes I just have fun w/ them, seeing how often I can get them to contradict themselves. I had one individual recently attacking sources he cited as 'right wing idiots' because he didn't bother reading past the first couple of lines.
Feel free to add your stories of truly ridiculous lines you've been handed or your own suggestions.
*PSH: Pants Sh!tting Hysterics