I died in lava and then tried to enter full-screen mode while the "You died!" screen was showing. On entering full-screen the buttons became grayed out and did not work. On leaving full-screen mode, the buttons were still grayed out and I was forced to close Minecraft through the processes tab.
Environment
Windows 7 64-bit
3.92gb ram
Minecraft running with 1gb of ram in 64bit java
Java Version 7 build 40 (build 1.7.0_40-b43)
Duplicate of MC-2835 - If you have not, please use the search function in the future, to see if your bug has already been submitted. If you could not find the original report, please comment with the keywords you searched for.