Iam so surprised no one else has ever thought of this! Whenever you drop hydralisk or drop zerglings into another persons mineral rush, take a defiler with you. The moment you drop use dark swarm (To hide your units) and burrow half of the units. Then when the kill off the hydralisks or zerglings that you left unburrowed, they will leave. After they get another mineral rush going. Unburrow. Works great against protoss and terran. And after you unburrow the defiler, have it eat a zergling or hydralisk and put the plague on his defense or overlords (according to his/her race) follow up with an attack through the very heart of his defense or use another overlord to drop in more units. This will work 90 percent of the time. And hey even if it doesn't, theres always a chance that your oppenent will get pissed and leave. So try it out. You want be disappointed!

CreepColony's Opinion: This requires some micromanagement, and if they have detectors nearby and see your burrowed stuff, this won't work.