Diablo 4 players are getting kicked out of Uber Tristram by one wrong click
The exit portal appears right beside the entrance and can apparently be clicked before it even finishes appearing.
The secret Tristram quest in Diablo 4 Season 15 has another little trap waiting for players, and this one can waste a pretty significant amount of farming. And no, the trap is not the bosses.
It seems like a good idea to warn you that the exit portal in the secret Tristram fight can apparently be clicked almost immediately after entering, even before its visual effect has properly appeared. If you are using mouse movement and happen to click in that direction, you can end up right back in Old Tristram instead of fighting the three Prime Evils you spent all that time summoning.
And unfortunately, you cannot simply walk back in.
Do Not Click the Portal When You Enter
The warning comes from players who have already learned this the hard way. The exit portal appears near the entrance to the encounter, meaning it is easy to click it by accident if you immediately start spam-clicking your way through after loading in.
The safest option is simply to wait a couple of seconds before moving, then click away from the portal once you can see it. And if you use a controller or WASD movement, you are safe anyway, since the accidental mouse click is what appears to trigger the portal.
The annoying part is that getting back into the fight requires another Wirt's Lucky Coin, and that means collecting the three Prime Evil Splinters and going through the entire process again.
How to Reach the Secret Tristram Fight
If you have not reached it yet, the encounter is part of Season 15's hidden Tristram questline. You will first need to complete the secret quest involving Deckard Cain's Journals, and that eventually gives you access to Old Tristram. From there, collect the 
Splinter of Hatred, 
Splinter of Destruction, and 
Splinter of Terror, put all three into the Horadric Cube to create Wirt's Lucky Coins, then use the coins at the fountain in Old Tristram to open the portal to Echo of Tristram or Uber Tristram (both names are sometimes used, but it is really the same place). That is where the three Prime Evils are waiting together for the final fight.
So, after spending all that time getting there, maybe do not immediately click in the direction of the exit portal. Just wait for it to appear, and then you can click away from it.
There is also a problem with the fountain that can sometimes eat your Wirt's Lucky Coins without opening the portal. If that happens, sadly there is no way around it, you will have to farm your Splinters again.
The secret Tristram encounter can reward the 
Hellfire Torch, a powerful Mythic Unique Charm, so accidentally throwing away your attempt is a particularly painful way to end the run. So when you enter Uber Tristram, wait a moment, and then move away from the portal carefully. And if the fountain eats your coin... well that is really just bad luck.