When minecraft has been running in the background for more than about 10 minutes my entire PC starts to die - input delay, mouse freezing, processes locking up. Usually I can fix it by ending the process, but I have had to power off my PC before I can do anything. Issue only occurs when a world is loaded.
Could be connected to the massive tick lag people have been experiencing (Etho's LP world has been hit pretty bad), maybe a memory leak?