I guess I'll take the downvotes and somewhat agree with them; probably not as far as to say a fireable matter for the first offense. Definitely a strong and unambiguous warning/disciplinary action (repeat offender, however? Fired)

Sorry, I get office pranks but there's no reason to be doing anything on an end-user's workstation without their permission or consent that isn't maintenance or security related. I guess I lack a sense of humor, but I've also seen too many instances where a prank on someone's machine resulted in severely unintended consequences and breaches of trust that rippled through the team.

Just not worth it, IMO.