Has anyone ever found a Potion of Constitution? After many games I have never seen one anywhere except in IWD2 item lists and the DK2 item browser.

No. This potion can only appear as random treasure at one certain location in IWD1 if you don't have the expansion installed.

I guess the developers just copied the items they didn't have to change from the IWD1 to the IWD2 item list.

I was wondering about this one as well. I'm glad somebody else asked because I keep forgetting to! :)