A friend on mine (notorious for beig extraordinarily tight) bought a cylinder from another friend of mine (notorious for being extraordinarily risk tolerant). Brought the cylinder for testing and half a L of water poured out. The cylinder is now used by the test centre as the demo for why you need to get cylinders tested.

IMHO the most prolific repeat offenders for water in cylinders are club compressors because dozens of people are using the compressor with varying levels of knowledge and understanding as opposed to shops and personal use where it is usually the same person with a working knowledge of the machine (I expect that this will be dismissed out of hand by some)