From Diablo Wiki
The Quality Well is an Easter Egg dungeon that has a chance to spawn in either The Old Ruins of Tristram, or otherwise along a specific spot in Old Tristram Road. Both spawn locations are noted on a map below.
The Quality Well is the sister dungeon of Development Hell. The difference between the two is that the Quality Well uses the same map tiles as The Cave Under the Well, and it houses Blizzard employees who worked on quality assurance (QA) during development of Diablo III. Unlike Development Hell, there is no superunique boss, nor are there any feats of strength or achievements associated with it. It is simply a rare easter egg.
The well has an approximate 1% chance to spawn. It will only spawn in Nightmare difficulty or higher. The Quality Well does not spawn in normal difficulty.
A video of a Barbarian exploring the well in Nightmare difficulty: