Very early on I was putting together my first database-driven web app and decided to use record IDs with leading zeroes - you know, because that would look neater. It was a long, long time before I came to know that a number with a leading zero would be evaluated as octal, which in turn meant that any number with an 8 or a 9 in it was invalid, which then meant it would be a match for any other invalid number and retrieve a seemingly random record.