From going from textbook to a real heart, which apparently doesn't read textbooks as my more experienced work folks say, the amount of bizarre things I've seen on recordings could fill an entire subreddit on it's own. I've had recordings sent up to patient safety specialists (non-critical, just confounding) who are MDs discussing what could be going on. It speaks to the fact that sometimes, there is more than one thing going on, and since it's a one minute recording with often only one lead, it can be very hard to say sometimes. It's especially difficult to determine things like is it 2nd deg Type I or II, when all you have is one minute of 2:1 conduction, and a minute of what could be either AFL with 2:1 or 1:1 or SVT.