I picked up four tanks at a Florida dive shop for my buddy and I prior to diving the Spiegel Grove, 2 120s for me, 2 100s for my buddy, which were supposed to be filled with 28% for our first dive to the bottom (140') and 35% for our second dive to 90'. The guy was locking up the shop when we arrived to pick up the tanks and was in too much of a hurry to get home to analyze them for us there. Fortunately I had brought an analyzer on the trip and we checked the tanks before we got on the boat. Sure enough, the labels on the 100s were switched. My buddy would have dove the 35% to 140' had I not been able to analyze the tanks and we decided to trust the shop's labels. Fortunately we realized the simple mistake, saving my buddy her life, and saving the shop from a huge wrongful death lawsuit.
However, in order to cut weight below 50 lbs in our bags, I stopped bringing the analyzer on dive trips where I'd be able to use the dive op's. That's served me well in most cases, with two exceptions. On a liveaboard at Cocos Island, the tanks were filled on the skiffs. The boat crew insisted on doing the analyzing because they didn't want passengers getting on and off the skiffs when we weren't diving. It wasn't a very reassuring system, as the analyzing crew would write the fO2s of the various tanks on the skiff on his hand, then tell us our numbers when he got back on the main boat, occasionally seeming to confuse them. Since the numbers were all in the 30-32% range, we simply set our computers to 30% to be conservative. There are other reasons why I won't be diving on that boat again.
Most recently, in Cozumel, I was surprised to find that the boat picking us up at our hotel did not have an analyzer aboard. The owner/operator assured me that he double-checked the numbers from the place where he picks up the tanks (dive shops in Cozumel don't fill their own) and I made the decision to trust him (he had a nice face). I had a few pangs of worry, but figured that if he had lied and I died as a result, he wouldn't be getting a very big tip from me. He said he normally doesn't bring the analyzer on the boat to keep it from the elements, but if I requested it, he would bring it the next time. Just in case, the next time I go to Cozumel where I plan to dive with the same operator, I'll probably lug my own analyzer along.