If it's really a crack, definitely toss it. I have what I thought at first to be a crack on the bottom of my 5 gallon glass carboy, but on closer inspection with a magnifying glass, it appears to be just a surface imperfection - like a string of molten glass strung across the bottom, or maybe a crack that had been remelted at the factory. If you really have a crack, I think that under the right lighting you would be able to see a shadow extending all the way through the glass - if you can't see anything except right on the surface, it may not be a crack. Of course, if you have any doubts, it's safer to toss it, and cheaper in the long run.