How common is it for relays to short out?

I was just wondering how they short out when they do? thanks.

The contacts ark from heavy loads and weld themselves together.

This is a good indication that a larger relay is needed.

https://img98.imageshack.us/img98/5093/0001277wr7.jpg

Not sure what contacts you mean. I can see the contacts inside the relay that open and close by coil could weld but would that be a short? The same polarity would be on both sides. I wonder if the coil sometimes gets a short and causes the two polarities to meet.

The external terminals(the part you plug in. look like plug) look too far apart to arc. thanks for your help.