Here's one I've done a couple times with great success:
If you work with someone who works in Word a lot, you can sneak onto their computer and go into Autocorrect. You can make it so that what they type gets changed all around. The possibilities are endless. Here are some random suggestions...
"If" becomes "If you think"
"please" -> "why doncha"
"I" -> "I begrudgingly"
"the" -> "the everlovin'"
"thank you" -> "it's about time"
"Sincerely" -> "Str8 pimpin'"
"because" - > "cuz guess what, B word?"
This will make anything they write come off as ludicrously insane and they won't know why. It's especially great for people who regularly write letters to customers.