Last year diving in Thailand with rented gear and an instructor as my buddy. Pre-dive check, everything's fine. Getting in the water and descending, fine. Keeping my buddy informed of my tank pressure, all fine, until next check 80bar, same as it was in previous check.
Buddy: really?
Me:really.
Buddy:OK. Continues the dive.
Me being a newbie (and apparently stupid) thought, well he must know what he's doing. After a while ascending for the safety stop at 5m. Breathing starts to get heavy. Pulling the instructors shoulder and signaling OOA. In comes an octopus (not the instructor's but another diver in our group). The instructor was a bit quiet at the boat trip back.
Lesson learned: pressure gauge fails during dive, dive ends there. And don't trust your buddy (even if he's the instructor) over you're own common sense.