Rx8 engine failure
 

We had the engine in our 2004 RX8 replaced around 92K, due to a crack in the casting allowing water into the combustion chamber. Is this common? The dealer indicated it was. Didn't see is discussed in the engine failure thread.

Maybe it's related to overheating.
Did it ever overheat?

The dealers aren't supposed to open the engines, so they shouldn't have been able to conclude exactly where the coolant was coming from. Coolant seal failures do happen, usually if the car is allowed to overheat.

Net result is the same though: engine rebuild/replacement.

No, this is certainly not common! In fact, I suspect a crack in the combustion chamber is exceedingly rare.

A crack in the actual engine casing is rare. I have only seen that in a rotary race engine, subjected to extremes. More common is a seal letting go.

Overheated
 

The car never overheated while we had it, however we purchased it used and could have happened then. The problem was that it was hard to start in cold weather and this was the diagnosis.

Hard start when cold (as in, the engine has been off for a while, shouldn't have anything to do with outdoor temperature) is indicative of a coolant leak into the chamber, so that makes sense. How they diagnose it was a crack and not a seal I'm not sure. Regardless, the fix is the same : engine replacement.

Engine failure
 

Yes, they said it was water in the combustion chamber that caused the hard starting, especially in the cold weather. I think they diagnosed it with a compression test, but as you say, that could have been things other than a seal. Would a seal cause water in the combustion chamber?

Yes.