As the title states, what are the conditions that need to be met for the Quality Well in Old Tristram (Act 1) to become available? For those who do not know what I'm referring to, there is a well that looks like a standard sprite in Old Tristram and sometimes this sprite will fill up with water and you can battle the zombie QA team from Diablo 3. I have yet to see the well filled with water and accessible, so I was wondering if I needed to meet some hidden prerequisites or if it's a truly random Easter Egg.
The only condition or requirement is that you must have completed the game on normal difficulty and be playing on at least nightmare, other than that, it is simply chance. If you keep reloading the area(by exiting and reentering the game), you should eventually come across it.
Please note that given the lack of information on these Easter Egg dungeons, the requirements above may not be entirely accurate, but thus far I have yet to see this dungeon, in game or on the internet, before nightmare difficulty.
You need to complete normal mode, but from there onwards it is purely RNG.
I have been farming gold in Act 1 and have seen and entered the 'Quality Well' many times, there appears to be no contributing factor to the well being active, it's always in the same place. There are multiple well spawn points within this area but the one you're after is located in the Old Tristram centre, opposite the old Blacksmith building.
I've had it active at the same time as another well in the starting area and by itself. I've also had it active with pretty much every combination of every other dungeon/cellar in that area so it doesn't seem to be linked to this either.
To unlock developer hell, select the 'A Shattered Crown' quest in Nightmare difficulty or higher, talk to Haedrig to do his quest and then travel out to the cemetery, get the way point and then see which instances have spawned. You're looking for 3x Defiled Crypt and the fourth will be Development Hell. Took me about 70 tries for it to spawn. Video of my find is in my comment below.
Development Hell is a crypt full of zombies and wretched mothers dropping very little, and one elite which gives you a Feat of Strength: "Smash! Jay, Smash!" (Kill the unburied Jay Wilson).