I have two; both of them could have been deadly in different circumstances.
The first one was during a dive in my Cave 2 class. We were supposed to swim a line that would eventually make a left turn and end, and we were to jump forward onto the gold line and turn right, further into the cave. The passage was at about 95 feet, and I was diving Nitrox. I was leading. The whole dive was an anxious one, because the line was tied into the ceiling, which made it difficult to watch. When we got to the left turn, the line went on and T'd into the mainline, which was NOT what had been briefed. I sat and looked at it, and I knew it wasn't right, and it didn't feel right, and I didn't want to do it, so I waited a bit to see if anyone would turn the dive on gas. Nobody did, so I swam forward, cookied the intersection, and turned right. At that point, I got an "attention" signal from behind, turned around, and saw that the line had indeed ended, as we were told. What I was following was SOMEONE ELSE'S jump spool line -- they had jumped FROM the mainline to our line, and I had missed the tie-in and the spool altogether. Now, of course, this was a class, so the instructor knew I was being an idiot -- but BOTH of my teammates had looked at the spool (one swam over to look at it) and neither had signaled me. Had we gone on, and had the team that put the spool in taken it out while we were there, we would have been completely confused when we came back. It was Ginnie, and the mainline has no obstructions, so we wouldn't have been in real trouble -- but in another place, we could have been.
About six months later, I did a dive in Jailhouse in Mexico. This was a path we hadn't previously taken, briefed by the guy at the dive shop, and would send us into a deeper passage than we normally dive there. But since the max depth briefed was 85 feet, I thought it was okay. We were again on Nitrox. Instructions were to take a jump that would go down and eventually come to a T, at which we were to turn right. I was leading. We put the jump in, went down, and swam along. It got deeper than briefed, again to about 95 feet, where I swam up to a weird place where the line was tied to the wall. There was an arrow (pointing out, I did verify that), then a tie-in on the wall, and about twelve inches away, ANOTHER tie-in, and then the line went off to the right. I looked at it and thought, "Huh, that's weird. Why would somebody tie the line of twice on the wall like that?" I turned right and went about my dive . . . until my husband's "attention" signal brought me back to see I had just swum past the T we had been warned to watch for.
Both incidents had the same quality -- a "Huh?" reaction, and a feeling that something wasn't quite right, but no ability at all to think it through.
I don't go to 100 feet on Nitrox in caves any more. Those two incidents scared the stuffing out of me.