Well, I've seen a lot of different things. I worked for a chain for 15 years, everything from automaticket machines, pacer to radiant systems. While computers certainly send up a lot of red flags to some things, somebody needs to look.
I've seen cashiers who have also been the ticket taker, simply resell the 2nd half of the ticket to the next customers. Thats quick money.
I've seen managers take the entire gross from special shows. Say a school calls and wants to bring 100 kids to shrek or something, the manager says ok, $5.00 a person, then never punches up tickets, and gets a nice Christmas bonus. Seen the same thing with concessions, charges each student $2.00 for a bag of popcorn and complimentary cup of soda, those items have a "loose inventory" so nothing gets rung in and your christmas bonus just got bigger.
I've seen concession attendants sell $9.00 combos, then later void them all and pocket the cash, (doesn't work very well,or too often, since every cup/tub was counted nightly).
I've seen cashiers void tickets and just pocket the cash.
I've seen managers just write of candy or other stock items as "Spoilage" to cover-up shortages.
I had an asst take $35.00 cash out of the dippin dot vending machine, then simply write off 10 dippin dot packages as spoilage to cover it up.
I even had an usher and concession attendant work together, the usher was going to pull cups/tubs out of the garbage/wipe them out, then the vending attendant was going to re-sell them. I caught that one before it happened thank god...
After the last movies started and the doors were locked, it was quite common for customers to want to get in late, so the ushers would let them in for $5 and just pocket it.
I had an usher who would "charge" for refills after the stand closed. Even though, no such policy existed...
Immediate termination of the involved employees usually got the staffs attention for a while...