I didn't realize that the usage of this phrase had become more general over time, but from the posts in this thread and Ghost's comment, I guess it has.
Until this thread, I always thought of "catching an edge" as something that mostly happened during a transition from turn to turn. Often this happened to new skiers, who, when told to "commit to the new turn" would attempt to do so when they were carrying too little forward speed through a transition and instead, were actually sideslipping downhill a bit during transition.
They would dutifully put their center of mass on the downhill side of their skis and engage their downhill edges, but because their skis were skidding ever so slightly downhill, and their forward speed and/or the sidecut of their skis was not enough, the new turn they were hoping to start wouldn't be tight enough. It wouldn't generate enough centrifugal force (
- joke from other thread) back uphill to prevent them from falling to the downhill side of their skis. Instead, the downhill edges of their skis would lock in and track across the hill as their center of mass got further and further downhill from their skis. So, unless they did a fast and massive stemming move to replant their downhill ski back on the downhill side of them, they would wind up doing a face plant to the downhill side of their skis.
At least that's what I always thought of as "catching an edge".
Also, it wasn't always a new skier that had this problem. Just like Yuki said, it could just as easily be a good skier who stopped thinking about skiing on some easy, low angle slope or catwalk. Catwalks were particularly good places to "catch an edge" because with their narrowness and traffic, many good skiers who don't want to stem to control speed in a crowd, just do prolonged low angle sideslips. Since they are mostly sideslipping and not actually moving in the direction their skis are pointed, and since their edging is minimal, a little chunk of crusty snow or rut can catch their skis while their bodies keep on moving downhill.