[Phlogiston]

I think that is the one that Sal sells.

[morzh]

Well...strictly speaking., X-rated cap is not designed to fail shorted. What it designed to do is to withstand large surges that could occur across the line. It may or may not short, and if it shorts, it may or may not blow the fuse, if the fuse has improper rating (not likely but....).
The Y-cap, on the other hand, is guaranteed to fail open and is used where the failure presents an electric shock hazard.

This is why if the cap is certified both X and Y (obviously a cap cannot fail both short and open on demand) it means, that yes it will fail OPEN (Y-rating) and will withstand large voltage surges (X-rating).