I don't use critical failures, except when it's really funny, appropriate, and causes no lasting harm.

In my current Pathfinder game, the gnomish Cleric decided that she wanted to memorize Shield Other to cast on the Monk/Sorcerer who kept getting nearly killed in combat. Looking at the spell requirements, I tell her that she needs to buy a pair of platinum rings that they each wear. She does so, and then goes to the Monk/Sorcerer to tell him that he needs to wear one of them from now on. The Cleric in-character tends to babble and explain things in an inarticulate, convoluted way, so she was going on about rings and a magic ceremony that would bind them together.

The Monk/Sorcerer is not understanding what she's talking about, so I ask him to make a Spellcraft roll to see if he recognizes the spell. He rolls a 1. I think for a moment, and then tell him, "You're not sure, but you think she just proposed marriage to you."

The misunderstanding was cleared up fairly quickly, but it's made for a hilarious running joke in the campaign.