Hmmmm. I don't really know either. I'd say that it's unlikely, but that there are particular circumstances that make it possible - the most obvious is Solo's suggestion. If you used a sensitive brand one day, and not the next, or you tested with FMU one day and not the next, and it's very early stages, then it's quite possible you get a BFP then a BFN.
There are probably other situations too - maybe drinking a lot of water before testing might dilute the urine, even if it was FMU.
If it was the same brand, FMU, but VERY early stages (eg before AF was due), it is possible that if the first test was very faint, then differences between test stick batches might means the first test was +, but the second didn't quite detect it. HCG levels double every couple of days. If a test is supposed to measure at 25, and actually picks up at 15, the next day levels would be only around 22-3, and a same-brand test that was of advertised sensitivity might not pick it up.
That's all I can think of

Wait a couple of days and then try again...
