I found this random topic to be pretty interesting: iTunes gift card codes: 16 alpha-numeric digits 16 characters, out of a base 36 (0-9 plus a-z) or base 62 (0-9, a-z, A-Z) . = 7,958,661,109,946,400,884,391,936 possible codes on an iTunes gift card. Now lets suppose there are 15 million activated iTunes Gift Cards circulating out there. That's a high number but even so, there would still only be a one in 7,958,661,109,946,400,876,391,936 chance of guessing one. Lets say further that Apples server handlng this is willing to do a thousand such requests per second. Perhaps it takes longer for each such request, but it can do a thousand in parallel. Given the numbers above, it would take 7958661109946400876391936 requests to guess one right answer. Given that there are about 31556736 seconds in an average year (365.24 days per average year), it will only take this inspired hacker 252,201,657,039,131,071 years. That's perhaps 252 thousand times longer than the remaining lifetime of our Sun. There must be an easier way to earn $50. http://www.freerepublic.com/focus/f-chat/1913984/posts

i would have to go against that. the possibility of that is much lower. the hacker doesn't have all the time in the world. he simply has the time it takes for that card to be activated by the clerk until he goes home and plugs it in himself. and average of 10-30 minutes.

Wow.. And everytime one's used we get better odds of guessing. But at the same time... We get a higher chance that the one we've guessed has already been used. So... Win-Lose.