As noted above, small purchases at gas stations are a fraud trigger, many perps will test stolen cards in this manner and then go on a big spending spree.

However, I wouldn't assume that your card is blocked as I have occasionally had my card rejected - the pump computers don't always work correctly.

Best policy is to carry 2+ cards. Also consider providing your issuer with your cell phone number as the "home number." I have received a few calls from Citi and CapOne after using my card outside my home area, sometimes automated, and able to avoid blocking by verifying recent charges. (Does anyone else find it ironic that they call your home tel to verify charges when you are traveling )

Foreign travel is hit or miss. I have never had a problem with Citi or CapOne, but I always call ahead with my dates.

If all else fails, remember that carrying some cash is always a good thing.