There are very large internal stresses in all structural components including glass carboys. When an object is whole and undamaged they all cancel each other out. When a crack occurs some of these stresses are relieved. In steel the structure usually bends a bit, a bend that may or may not be visible but that is because it is ductile.
Glass is very brittle with nearly zero ductility at normal temps. If in fact this is a crack you see I’m surprised it hasn’t failed catastrophically already.
I’ve personally seen very severe hand cuts requiring hospital emergency room care from a breaking carboy and would absolutely discard that Carboy again IF it is in fact a crack you have detected. Losing a batch of wine would be my very least concern. I’m so averse I have purchased heavy duty milk crates for every single glass carboy of mine. I have over 30 so that was no small expense.
Plastic has its own potential problems for long term wine storage so for me it’s either a glass Carboy or a stainless steel keg.
I STRONGLY recommend that all glass carboys be paired with a milk crate or some other shield between the very brittle glass and any hard surface like concrete, stone, metal, etc. A HD milk crate has the added benefit of making it easier to carry a loaded Carboy around, if one has the physical strength to do so.
A Carboy that fails can (read that as it usually does)explode, the risk is NO joke.