dtgreene: Here's another interesting question:
If the game provides a clear warning before the point of no return (complete with a message like "Are you forgetting something?"), is it still a serious design issue?
Breja: As long as there is somewhere before that some sensible indication of what that item might be, then I'd say no. It's still not something I'd applaud, but I wouldn't call it quite as broken anymore. But if there is no indication before, and there is no way for you to know that some totally unremarkable, never mentioned item among plethora of others is important, then just asking me "are you forgeting something?" changes nothing.
The game I am thinking of, Wizardry 4, does provide some indication. Specifically, on the second level, if you explore it thoroughly, you will find the gates to hell. This is *not* something you will miss, and even if you can't make it there, the encounters near there are suspicious.
Note that you can't actually do anything at that spot when you first reach it; you need to backtrack. Fortunately, there is a warning right before the point of no return (as I mentioned earlier), and actually getting back to the bottom of the dungeon before that point is just a single cast of MALOR. (Getting back up has to be done manually, however.)
It is worth noting that Wizardry 4 expects you to thoroughly map each floor; there is, in fact, a puzzle whose solution involves a certain detail you might notice if you map everything.
By the way, for this topic, Wizardry 4 is best treated as an adventure game rather than an RPG; there aren't any character or party builds, and you don't have much control over combat anyway.