Is autocomplete really necessary though?
I also suspect autocomplete to be the leading cause of a particular set of subtle bugs - ones where the words on the page seem right but the function chosen has a surprising and uncatered for side affect.
When you reinforce to people that it is OK to code blindly ahead, grasping for the right calls at the last second you will end up with poorer quality code (although, it may be typed faster).
I am a believer in using as descriptive identifiers as possible to make the semantics of your code as clear as possible. If this gives rise to 5-word+ identifiers, so be it, providing it is done tastefully (and not for verbosity's sake).
As typing 10+ characters per function call/variable call can get tiring, lack of autocomplete can leads incentivizes the use of shorter names which runs counter to this philosophy, and leads to code being harder to read. (IMHO)
Some of the worst D'oh moments I've had came from before I added tab completion to my .vimrc, because it almost completely eliminated errors that my typos introduced.