I'm a law-abiding person and don't suggest overstaying the 90-day period, but I'm curious to know if this is ever enforced. Most of the time, the immigration officials don't stamp or scan my passport when I enter the EU. Exit inspection is even more haphazard. Accordingly, it seems like there's no risk of getting burned by SIS. I guess if someone enters the Schengen zone a lot through airports that stamp or scan consistently (e.g., MAD), it would become obvious if that person were around too much.