I've recreated this 3 times in a row so I felt I should report it. if I add, then immediately remove a position marker it crashes, and here's the error report:
http://pastebin.com/mud8q7if
EDIT: Oh and before you say it, I recreated it with and without Optifine :)
EDIT: Got it again in another circumstance. It seems to be happening when you remove a position marker when there are two. Once you remove it to 1, it crashes. Perhaps the path viewer gets messed up??? just my two cents, I'm probably way off! Here's the paste from that: http://pastebin.com/56XLpc4Y
EDIT: OK 100% recreatable, If there ore 1 or 2 position markers, and you remove one, it crashes. Optifine or no Optifine.