my sister in law got me and my wife good.
she created a letter stating that a couple of students (i am a substitute teacher) had complained to a school's principal that i had said some inappropiate comments to them, and that I had to go to court, etc.
i read the letter first, and knew about half way through that it was a prank. i chuckled and said nice try.
well, my wife came in about 5 minutes later from getting coffee, and knowing that she falls for this kind of stuff easily, i gave her the letter all serious-faced like.
Her face went from "what is this?" to "fucking shit" in a heart beat.
She just stared at me and looked down at the letter, and then stared at me again (later she said she was thinking what the hell stupid crap i would say... i say some stupid things occasionaly, but i'm not lame enough to slip in front of students).
Her mom asked my wife something about her day, but she ignored her and kept on staring at me. I started busting, but she kept on staring (she still kept on believing it because she says that i laugh sometimes when i am nervous and do stupid stuff, which is true). Then one of her other sisters says something about april fools, and the jig was up.
i had an awesome laugh, and my wife did as well

good times