The other day my credit card was declined for a ~$2 purchase of envelopes. The cause was purely bank incompetency, not a lack of funds. I'm a customer of a large number of banks in US and Canada and I can tell you that banks made errors like this all the time. Additionally, since my money is split across around 30 accounts, it wouldn't be too surprising if one day I use the wrong instrument to pay for something, giving the illusion that I no money (although I haven't actually made this mistake yet).
I just had to comment on this because it's definitely a mistaken impression to think that a declined card has anything to do with a lack of funds -- almost always, this is caused by bank error.
There was a "What Would You Do" episode about this. Presumably, a guy and a girl are on a first date, or newly dating, and they're waiting on line to pay for something. His card gets declined for a relatively small purchase and he starts worrying. They, of course, are actors, but they wanted to see what people around them would do. Someone told the girl to get out before she's in too deep, because she shouldn't have to deal with her man having a declined credit card. It went on to explain how cards are usually declined because of some sort of computer error, either from the bank or from the store itself.
My AMEX was declined once while I was grocery shopping. I was doing my bulk shopping and had a pretty hefty transaction (~$100). I scanned it three or four times and it said 'card declined' each time. I had just paid it off in full and was no where near my limit, but I still had that feeling of 'oh shit!' I was more concerned that someone had stolen my cc and had run up a ton of purchases. The guy behind me laughed and asked me if I had too good of a weekend (it was a Monday). The cashier took my card and swiped it on her side and it went through just fine.
Moral of the story is: seeing a card being declined means nothing. But, with the stories on this page, typically it means people don't know how to budget.