The piston crown can also get burrs on it as shown in the diagram but is much less common.

The ring sticking is a much more serious problem. Some builder relive the rings around the exhaust on new pistons, some do it once the piston is run in. Personally I do it after run in. Then keep on top of it for the pistons life.

If the ring is allowed to stick, not only does it wear the ring badly, it wears the exhaust bridge and forces the piston against the the inlet side of the bore. The cause of the burring is due to the large unsupported area and the heat.

Run a file over the exhaust side of a used pistons crown and then over the inlet and you can feel how much harder the exhaust side has become.
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If I have to take the carbs off once more...