I have been doing the RCM Ear Training online course daily. One form of exercise is to hear a very short melody, then play it back (which is recorded by the browser/webapp), then replay it to check accuracy against the displayed score of that melody. Well, today, whatever I played, the recording was played back one whole step up in pitch!
I was first surprised, then bewildered. After several instances, it became annoying. I had to get to the bottom of this. So I sang the melody as I played, to find out my voice was raised as well, so the pitch raise is on audio (rather than MIDI). It racked my brain to think of any software glitch that could possibly cause such a perfect pitch raise. Then it finally dawned me when I noticed the calendar date on the menu bar. ROFL
Now let's hear your best April Fools prank involving a piano, or music!