Under conditions where endermen are unable to place blocks, they can get stuck forever. This is a serious issue, as it hurts the performance by filling the world with endermen. Here are most of the conditions in which an enderman will get stuck forever:

• Endermen holding anything can get stuck above the Nether's ceiling forever, as they cannot place blocks on bedrock
• Endermen holding anything can get stuck forever, if they pick up a block and get trapped in a place where there's no valid position to place a block
• Endermen holding crimson/warped roots/fungi can get stuck forever, if they get trapped somewhere in a Nether wastes or basalt deltas.
• Endermen holding cactus can get stuck forever if they teleport into a cave with no escape
• Endermen holding flowers can get stuck forever if they teleport into a cave with no dirt surfaces or escape

The best way to reproduce this is to set the world border to 128, go into the Nether, reach the ceiling, place tnt floating 1 block above the bedrock (so endermen can't walk on top of it), and spawn endermen. When the endermen pick up the tnt, they will be stuck forever. (note: the world border only serves to confine the endermen within an observable area; it still works without the world border)