That pretty much sums up what I wanted to say, on each point. I'm fortunate enough to not need visas to most places I visit. Putting myself in the OP's shoes, I think I would have caught it if I was taking a flight between two cities in the same country but transiting a third, but not in his situation of going between two Schengen countries via a non-Schengen country. I realize that's a bit ironic, but I'm just sharing how I think I would have reacted.

Another aspect to this is that even if many of us would not have caught this situation ourselves, even if we checked all of the relevant websites, there's the issue of blame. When something like this goes wrong, there seems to be a tendency to want to assign blame to someone, and more specifically, someone else. Notice how some in this thread seem to be stretching to assign blame either to the embassy that issued the Single Entry visa, or to the airline. Sometimes, things just happen. And sometimes, the person that bears responsibility would have had a hard time to prevent it from happening.