...the "are you sure you want to do XXXX?" question. Even used when they are on the right path, it can drive player's batty. ^.^

"Are you sure you want to drink two potions right after each other?"

"Of course. This way I'm protected and invisible. What's the worse that can happen?"

"I don't know, let me roll to see."

Ever hear of drug interactions? The same can happen with potions...you think your going to be invisible and protected, but you end up violently ill...Fort save to keep from vomiting up the potions before they take effect?

And then there's the little known side effect of drinking too much cure potion at a time...Fort save or have a bad case of the runs for as long as the DM finds it amusing.