This happens to me quite often but if I think a bit about it, I'm not 100% sure either why they're completely wrong.
If you want to get into the conversation (not recommended as it gets ugly quite often when people are completely sure of something * caugh...republicans... caugh*) it's actually better to argue on why they're wrong to be COMPLETELY sure rather than argue the opposite point you might think is right.
They will have far less argument to explain why their point is completely right than to show that yours isn't right either.
All of the best republicans I have known are their own biggest critics. All of the best democrats I've know have been as well.
The problem is that its the most cock-sure on either side that get into the intractable arguments with each other, or worse, rush into action so sure that they are correct that they never even consider the unintended consequences.
Sadly, it also makes them much more likely to be elected.
Anyone who is capable of getting themselves made President should on no account be allowed to do the job. - Douglas Adams
If you want to get into the conversation (not recommended as it gets ugly quite often when people are completely sure of something * caugh...republicans... caugh*) it's actually better to argue on why they're wrong to be COMPLETELY sure rather than argue the opposite point you might think is right.
They will have far less argument to explain why their point is completely right than to show that yours isn't right either.