Steps to reproduce:
- Build a bridge sideways (i.e. holding shift+S+D or shift+S+A and right mouse button, while pointing at the side of the block you are standing on).
- While bridging, you run out of blocks and switch from the empty slot to a new, full slot using the mouse wheel. (Do not release any keys or mouse button).
- Fall to your death.
I can reproduce this in creative mode and survival, in 1.4.7 and 1.5. It is much more likely to happen under increased latency conditions (ping). (I cannot reproduce it in singleplayer, presumably due to the lack of latency.)