The worst I ever did was in the early eighties, programming a silicon wafer probing robot for a wafer fab - included a subroutine that would, about once a month, blank out the controller screen and display something juvenile (since that's what I was back then) like "This incredible software developed by..." for about 5 seconds. Without loss of data, of course. No harm done.
Context is everything. If the target of the above prank is a good buddy and they have a history and an understanding about practical jokes, then , yeah, it's funny. If it's just an innocent colleague, then you've definitely crossed way over the line.