I recall a couple of bad calls of my own, though got away with it. Took a pal on a winter trip to Snowdonia, and as he'd been saying he wanted something a bit more exciting than he'd been doing local rambling group suggested Crib Goch, though warned him it was a bit airy, but he said he'd give it a go. We lent him an ice axe and off we went. Fairly soon realised Crib Goch covered in 6" of melting slush was a bad idea, and the axes were of no use at all. Anyhow he did pretty well, but when we finally got onto a more normal path he looked me in the eye and said "That's the worst thing I've ever done in my entire life, and I never ever want to do anything like that ever again" ! To be honest we shouldn't have done it at all in those snow conditions, never mind with a novice. Oops!
Another bad call was taking a bunch of (experienced) students on a particular route through a cave involving a stream section. We got to the stream and my mate, like me the other veteran caver, commented that the water was a bit high, but I felt it was OK we would only be in the stream for a relatively short bit for the planned trip, and not the fiercest bit of the stream at that. It quickly became apparent that this was a bad decision, but it wasn't safe to turn back as in high water conditions you are better off going upstream (as we were) rather than downstream as the water can knock your feet out from under you. The highlight was a climb up a 5' waterfall - simple in normal conditions but on that day it was a horizontal jet of water going some way along the passage. We had to climb up further back and traverse along. If anyone had slipped they'd have gone. Anyhow we got away with it, as luckily the lads were fit and strong. Many later some of the former students had joined our club, and one commented about the "fantastic trip in the streamway we'd taken them on when they were students". With a slightly less strong team we could have lost someone, though perhaps if I'd then known how very capable their group was I'd have thought it bloody good fun if slightly risky.