While playing on my 1.7.5 server I was using my nether portal numerous times and realized that when trying to open a chest (or any other inv block) with a block in my hand, the block would be placed even if I was in its way. I'm not completely sure if the portal has to do with it though.
Please force a crash by pressing F3 + C for 10 seconds while in-game and attach the crash report (
[minecraft/crash-reports/crash-<DATE>-client.txt|http://hopper.minecraft.net/help/finding-minecraft-data-folder]
) here.