[Always *]

What are the planning to do about it? If you were $50 off, and you let them know it must have been a mistake because you counted and double checked, then what's the big deal? If this isn't a recurring issue, then they aren't likely to do a lot about it. I know $50 seems like a big deal, but if you talk to them and promise you have no idea why it's not there and offer to help find it (e.g. was the cash ever out of your sight, was it possible someone stole it? Is it possible that you did miscount it?)

Things like this happen with cash registers and a lot of places have a margin of error. By the sounds of it, $50 is outside of the margin of error, but still... I am sure it can be explained and that you should just be extra careful in the future.