Once a ghast has randomly floated into a column of falling lava, it becomes stuck there for a long time (possibly permanently). It will move up very slightly but will not move horizontally at all. If it can be nudged out of contact with the lava, it resumes its normal wandering.
Steps to reproduce:
1. Go to the Nether and scout for a location having a large, open area with many columns of lava dropping from the Nether ceiling.
2. Wait for ghasts to spawn and watch for one that flies into a lava column.
Expected result:
The ghast floats through the lava as if it weren't there.
Actual result:
The ghast floats through the lava until it is just about to exit contact with it. It then stops making horizontal progress as if impaled, and slowly works its way up the column.
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Affects 1.9.0
Because there is still exists there is stuck ghasts
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Affects 1.10.0.5 Beta version
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Affects 1.10 full release
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Affects 1.11.0.8 beta version
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Version 1.11.4
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that gives unfair advantage because it can give you the "Return To Sender" achivement
That also affects 1.16.50/1.16.40/1.16.61 and 1.16.100.xx beta
lizking10152011
This also causes extreme lag in the 1.16.201 Hotfix on both Realms and Custom Servers when Ghasts become stuck in lava falls and can bring things to almost a screeching halt. Tridents with Loyalty will usually automatically return to player's inventories without being thrown if attempted to be used to kill the Ghast stuck in lava, unless it is held back for at least 10-30 seconds before throwing it when this happens with the Ghast. This should not happen and the Trident should be thrown immediately upon releasing it.
This also affects other mobs too when this occurs on Realms and Custom Servers when a Ghast becomes stuck in lava, and can bring them to almost a screeching halt, making their movement extremely slow or at times making them stationary.
Auldrick
@unknown: I have removed your comment because it describes one or more different bugs.
Please submit a separate report for each bug. It seems quite unlikely that a problem with ghast pathfinding has anything to do with lagging the game, but if you're finding that it does, that's a different bug that needs a different report. (If you create one, be sure to describe how you were able to prove it to yourself that the ghast is to blame, not something else). Furthermore, the effects of lag on tridents and "other mobs" (which ones?) are not relevant to this report at all, as only a ghast can fly into a lava column and become impaled by it.
Auldrick
Still affects 1.17.0.
In retesting this, I thought of something that might explain virtually everything that's happening, something the devs may be neglecting because it's neither visually obvious nor obviously connected with the symptoms of this issue as I originally described them.
I had long ago noticed that not all ghasts get trapped in a lava column. I dismissed it as not remarkable, since it's common for a bug to involve random and/or unseen factors that trigger it. But this time I happened to be watching two ghasts, one right in front of the other, as they flew into a lava column. The first one went through, though I noticed it slowed just a little while it was in contact with the column. The second one came to a complete halt and never made it out.
The thought that occurred to me was that if contact with the lava is slowing the ghasts' forward motion by a fixed amount, a slightly faster one might be slowed but eventually get through, while a slightly slower one might lose its forward motion altogether. From there, secondary effects would follow: They would no longer be able to turn (because of how turning is implemented as a modification of pathfinding), and although I'm not able to explain the slow ascent, that might also be a result. So then rather than looking for something that could trigger random ghasts to behave differently, the devs should be looking for whatever causes any ghast to slow down as they pass through a column of lava.
Makzevu
Mobs that encounter lava always attempt to swim upwards regardless of their pathing goals, unless they are undead. I suspect that not only were ghasts not added to this exception, but they are also given swimming physics when colliding with lava, causing them to move slowly.
The scenario where ghasts get completely stuck is probably started by a ghast's pathing goal being set in or next to lava (at least enough space that the ghast's hitbox is clipping into lava), then the ghast flies to the correct x and z position, and proceeds to swim up and away from their y destination due to being in lava. Pushing ghasts in this state will result in them wanting to fly back into the column, remaining stuck. In my tests, removing the lava caused ghasts to travel down and eventually start pathing again, suggesting that they were also trapped in their pathfinding until their goal was reached (MCPE-49607). In fact, their ability to fly down might be getting slightly overpowered by their urge to swim up, causing them to move up slowly rather than quickly.
GoldenHelmet
I un-restricted Timothy Chapwick's comment below because it just describes impacts of this bug, and it is corroborated by MCPE-171681. The video linked on that report very clearly shows the extreme lag that can accompany a ghast getting stuck in a lava column. A plausible explanation is that this bug triggers MCPE-147882 when the ghast has a target.
Auldrick
Still happening in 1.21.2, although it may be that ghasts have a smaller chance of getting stuck in lava columns now.
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