This is all vanilla. Client is 1.8 on Windows 7. Happens with current Java as well as 64 bit best java 7.
The bug normally is: I open a map and I'm not on it. We share our maps, so it's only happened on a shared map.
The green pointers for where other players have maps in item frames always appear. Edit: with tonight's daily restart of the server so I could play, this time, only the "first" green pointer shows up.
Other players will appear on it.
When holding adjacent maps, I may or may not properly show as a moving white circle at the edge, meaning that I occasionally don't show up on any maps at all.
One time, only one map didn't work.
Another time, I rendered as the circle when outside a map then as I turned into the pointer and began moving on the map I'm holding, the pointer disappeared underneath the map itself.
Once or twice, having everyone log out at the same time fixed this problem. Now that we've played more and have more maps, this doesn't work anymore.
Stopping and then restarting the server always fixes the problem.
Steps I've tried to fix which do not work at all:
Throwing away / dropping the map and picking it back up.
Leaving the map in a chest then logging out and logging back in.
Server is fresh ubuntu 64 bit install on 4 core / 8 thread i7-2600 CPU @ 3.40GHz with 8 GB of RAM. It's dedicated to this purpose & never shows a load.
Initially, the server's java was open-java like you get from apt get install. Now it is oracle java 7 and while does seem to perform better for the clients, I still am surprised when the map DOES work and my usual expectation is that the map won't work.