Best audio prank ideas

I was going to add this to another thread - but this deserves its own.

I have a plan to use a spare spot in the switchboard / fuse box to power something I can place under a floor, or above the ceiling.

It will randomly make the sound of a cat.  Maybe activated when it is dark?

At some point, I won't be here but someone new will be.  They will switch all the RCDs on.  Let the fun begin.

"Do you hear a cat?"

I know how the kitchen down lights are wired - I could just add it to that circuit?  There is a secondary switch which isn't wired both ways and that has caused me grief thinking that the lights have stopped working.

I know people have a compulsion to have a switch in the off position - no, that won't be off - it will be cat on.