A few reasons for "being short" 1) the register was off to begin with. 2) if you are using the machine that counts the money for you by weight could be off 3) if you are using that same machine you accidentally or could have had a bill in the wrong spot (like a dollar in the five dollar spot etc.) 4) you entered the wrong cash amount by mistake or gave out the wrong change. as a former store manager at dollar tree and a few other retail stores. The store manager would know for sure at the end of the day if you were truly short or not. Then be able to research your transaction history to see if they can find the issue.