Depends upon what you mean by a crack. If the meaning is that there is a chip taken out of the back of the mirror then it's no problem. The chip and it's hole won't affect anything. If by a crack you mean a split that's still going into the glass, you have to look at where the split is propagating to. If it is just looking like it's going to quickly pop out a chip on the back then accept it but if the split looks like it's going to delve deep into the glass, reject the mirror. It's kind of like a rock star on your windshield. At the beginning, it isn't much but all of a sudden, one day it gets the idea to start relieving itself. If the flow of the glass is such that the split wants to go into the glass then you're in trouble. One mirror that i have seen was dropped and a large chunk was popped out. That was no problem as no cracks were propagating from that area. However, another part of the mirror had an imapact also and a split started propagating in from there and it looks like the split is going to take out about 1/2 of the back of the mirror if it doesn't take a dive for the front. That's the dangerous split in the glass. P.S. Please in the future hit the button only once. Thanks.