when you enter a nether portal on multiplayer, you can immediately go back through without going out and back into the portal blocks. if you're lagging this can lead to an infinite loop where you can't get out of the portal.
MC-92303
Nether portal bug
Resolved
Duplicate
0
xander
(Unassigned)
Unconfirmed
Minecraft 15w44b
Retrieved
Thank you for your report.
However, your ticket has been resolved as a duplicate.
This issue has been linked to the original bugreport: MC-90605. You can find or add further information on that report.
Please search before reporting any bugs, as it's likely that one exists already.