On any TI-83+ based caculator create a new program:

:Lbl 1
:Input "", Str1
:Disp Y1*.9
:Goto 1

This program, when run, will present the hapless user with a prompt identical to the home screen. However, all input is instead passed (in a roundabout sort of way) to the Y1 variable, where it is evaluated and stored. You can then do what you like with it. In the above example, the user receives the correct answer multipled by .9, this being set in line 5. Two plus two will be thus be evaluated as 3.6. Naturally, line 5 could be changed to do all manner of things (random numbers, pi every time, etc.)

If you are feeling in an exceptionally cruel mood, merely set PrgmPUNKED to run on startup (using the TI Startup app), sit back, and quietly honk in amusement as the victim's calculator gives them very slightly wrong answers. I once performed this very atrocity on my innocent unsuspecting calculus teacher, on an overhead projected TI-84+SE no less. She figured it out with admirable rapidity (after all, she does teach calculus), but for roughly five minutes of class time I had to fight to keep a straight face as her calculations failed to match the answer key. She was a good sport, thankfully.

note: while not technically hurtful in a physical sense, some very real academic damage could be done to someone if one were so heartless as to do this just before a major test or exam. I deny all responsibility for any pain and suffering inflicted with this method.