Those were the days...
...trying to dream up novel ways to annoy your fellow students at univeristy!
I remember writing on TSR that hooked into the keyboard interrupts and, at random intevals, would replace the keys codes the user was creating with a predefined message so it looked like the user had typed the message to hilarious effect (I wish).
And then I discovered how to control the video hardware and created a TSR that would turn a text mode screen upside down by switching the page the video hardware was displaying and then frequently copying the default page to the displayed page (OK, it didn't turn the individual characters upside down).
Fun days indeed.