I had a client who refused to pay for his PC after our shop had done repairs, so just before he came to pick it up, I filled the case with loose stale popcorn. I'm sure he could smell it when it heated up.
Another time I had an endless looping batchfile running silently, scattering endless blank text documents across the desktop faster than you could delete them.
Another time I had an endless looping batchfile running silently, scattering endless blank text documents across the desktop faster than you could delete them.