When thinking about the Validate Level functionality due to it being mentioned in another thread regarding the skill count, I remembered that it warned about missing hatches/exits and thought to see what would happen if I put a single preplaced lemming in a level with an exit and no hatch in a level whose stats were not changed from the default save 20 out of 40 lemmings. I expected I'd have an unbeatable level and maybe Validate Level would complain about it. I definitely didn't expect what behavior this actually caused:
The player recognizes the level is only capable of dispensing one lemming, so it shows level stats of 1 lemming, 1 to be saved. However, when it checks the results at the end of the level (assuming the player solves the level successfully according to the displayed stats), it disagrees with itself as to whether or not you beat the level: the displayed stats are consistent with beating the level (1 lemming, 1 saved), but it provides a message consistent with failure: "A little more practice on this level is definitely recommended!" Internally the game appears to treat this as a successful solve: the autosave successful replays feature will save the replay, and if the level is run through the player and not the editor, you will receive a checkmark for completing the level.
The attached level demonstrates that the player will adjust the level's obviously bad stats in an attempt to make it beatable. I suspect the mismatch between the result and the end of level message is due to the message being based off of the level's original stats rather than these adjusted stats.
The Validate Level functionality only mentions that the level is missing a hatch, which is true, but maybe a little vague. On the other hand that is the root cause. So while the Validate Level could probably be modified to detect this particular issue more specifically, it's probably fine as is for this particular scenario.
EDIT: This is specific to levels with the ability to dispense 0 or 1 lemmings (including preplaced). I was also able to replicate the behavior using a hatch set to spawn only 1 lemming. Validate Level will display an error that the level's save requirement is too high in this case rather than that the level is missing a hatch.