On the TI-83+, the calculator of choice at my school, there are several ways to scare people who let you touch their calculator:

This one doesn't work all the time, and has varying results from clearing the RAM to displaying lots of funny characters and breaking in a very stylistically way and clearing the RAM. The results vary, but it can be quite entertaining:

Press the following buttons:

Enter the following:
Equ-Str> (Find this in Catalog (2nd+0))
Y1 (Vars, Y-VARS, Function)
a comma (",")
Str1 (Vars, String)
Then hit enter a bunch of times. Then try accessing different screens like Graph and stats. Everything you cared about was archived, right?

Some fun Asm programs and how to make them:

Hit "Prgm", and make a new program, name it whatever you want The first line must be the AsmPrgm token (Find it at the top of catalog) Then enter the rest of the program, and to run it quit out of the program editor and enter Asm(prgmNAME). (Get the Asm( token from the catalog as well, and then for prgmNAME simply select your program from the Exec menu of the program list)

A program to "test" your LCD and make it display blue lines (by overheating the display)
Wheeeee pull out the batteries to quit it.

A program that sets both the horizontal AND vertical graph split modes:
Go to the mode menu to fix the problem.

Set the program to self-test mode:
Go to the mode menu and type "S"

And of course the classic standby to mess people up: Change their contrast all the way up or down (2nd+up or down arrows a bunch).