I've had really random declines from time to time for no apparent reason--sometimes for very small amounts. I make a point these days of having backup cards across multiple networks (i.e. both Visa and Mastercard).

Also spare ID and cash. The one time I broke this last rule on a trip I was nearly really screwed. (I lost my license and the hotel didn't want to let me check in. They only relented when I showed other ID and was able to make a just large enough withdrawal from an ATM and pay cash.)