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
Thanks for the heads up. This is a pretty good message to map makers who wanna use custom potions in their maps.
Oh and I once got killed by a potion set to 127.