Smaller explosives, like M80's, when contained in a pipe can exert a ridiculous amount of force. It is easily enough to burst pipes - especially in newer buildings that use ABS for sewer drains. This can really fuck up the building's sewer lines, depending on how they're set up. If a bank of toilets is all manifolded together, you'll blow a hell of a lot of water out of every one of them, usually on both sides of a wall.
Easy to get caught though. Best time to do it is when there's a lot of people taking a shit. But also easiest time to get picked out of the crowd as the person responsible. Not worth the jail time for some cheap LOLs.