They will notice that something doesn't balance off.
But how can they know who got a 50$ instead of a 20$. First of all, there shouldn't be any 50's in the machine. First of all, they will think that an employee kept some money.
Then, if they trust the employee, and try to figure out who got a 50$ instead of a 20$, since they will be 30$ in the hole, they don't know where the 50$ was in the whole stack of 20's in that machine.
I'm sure that there is about 10,000$ per ATM machine all in 20's.
When an employee fills a machine with 500 20's lets say, and if a 50$ was put in accidentally somewhere in one of the 500 slots where a 20$ should be, they have no clue in where it was placed in that stack of money....so they don't know who got the 50$. If they knew in what position the 50$ was in, then they could find out, but since there isn't supposed to be other than 50's in that machine, they have no clue where it was, in the middle, at the beggining, or at the end of the stack of 20's.....
If they would have given me lets say one extra 20$, then the person after me would have had 20$ less, then they would have found out who got more.
I think I got away with a perfect crime.