I've called a couple dives due to bad conditions when we arrived at the beach, frequently I'm the "senior" diver and I feel rather responsible for my buddies. I've also called a couple of dives "mid-dive" due to getting cold, and once just because I discovered how uncomfortable I was doing that dive with the buddy I had (who happened to be an experienced instructor... I just did not have much confidence in my safety that day with him)
On the other hand, I'm the last one to call a dive just because the vis is lower than usual, or the topside weather is bad etc..
I also had one dive I should have called (maybe more than one). My air tasted "funny" but being a shallow dive I figured I'd see if it caused problems before calling the dive... wound up with a mild headache, but my buddy agreed that I should have thumbed it. I informed the boat crew when I boarded the boat and was told "there can't be anything wrong with our air, we use an oiless compressor." I did make sure to inform them away from earshot of any other passengers, as I assumed it was a fluke, but I did want them to be aware of the problem.
Aloha, Tim