Reference equality's inability to survive a persistence round trip is the big killer. And sneaking in a round trip in a process that used to be fully in memory is where dragons lurk.
Sometimes you are seeing if a constructed instance equals an existing one. I would presume that is rather common. Especially with points, a basic collision system is often basically an equality check. Right?
A "point" would be a value type and you wouldn't use reference equality or inheritance for a value type.
But if this were a game you might have a "BigBoss" instance than inherits from "Enemy". If you had one boss, you'd only have one instance of it, and you would use reference equality. If you round-tripped and ended up with 2 instances of the same boss then you're already in trouble.
