"Well, what if you're wrong?"
I usually see this question in response to someone who argues strongly for any kind of position on anything.
I really wonder why people ask this, there's no purpose to it other than using it as a clumsy attempt to plant seeds of doubt. Who cares if I "might" be wrong? What if you're wrong?
The same seems to apply to pseudo-philosophical questions like "What if everything is an illusion?". What if we all die in five minutes and this conversation was pointless (though if someone's asking you this, it probably already is)?
Instead of asking "what if you're wrong" why don't you actually give me a reason to think that?