Minecraft 1.5.1, Minecraft 1.5.2, Snapshot 13w26a
Operating System: Windows 8 (amd64) version 6.2
Java Version: 1.7.0_17, Oracle Corporation
Instant health effect set to a 29, 61, 93, 125, 189, 221 or 253 value kills any player (in either Survival or Creative mode!) without showing any death message. In addition, upon switching from/to the fullscreen mode (by any means) when this death screen is displayed, the buttons "Respawn" and "Title screen" can not be pressed anymore (at least in Singleplayer), and all you can do is exit the game. In singleplayer, the next time you open the world the buttons can be pressed again, but still until you switch the window mode.
Example steps for reproducing:
1. Make a cheats-enabled world in singleplayer.
2. Use the effect command: /effect <playername> 6 <any_value> 29