stuff happens

A phrase used by all to justify various kinds of strange occurrences. Used when the phrase "shit happens" isn't quite appropriate.

teacher: "your car caught and fire and your homework got burned??"

student: "yeah, well, stuff happens"

stuff happened

A discreet way of referring to various sexual acts, often used by women. It certainly refers to more than a quick kiss, but can denote anything from a cloths-on make-out session to full-blown intercourse. The lack of explicit detail is intentional on the part of the user of the phrase.

Girl: "Well, Jim and I ended up sharing a sleeping bag on that camping trip, and stuff happened. Now we're dating."