This cause for this bug is an endless loop in AutoCompleteWindow.py, line 121:

114 selstart = self.completions[cursel]
[snip...]
121 while cursel > 0 and selstart[:i] <= self.completions[cursel-1]:
122     i += 1
123 newstart = selstart[:i]

The case where this loop becomes endless only arises when the same completion item appears twice in the completions list, thus self.completions[cursel-1] and self.completions[cursel] are identical.

This happens with the os module because spawnv and spawnve appear twice in os.__all__.

Solution:
1) Fix the potentially endless loop (add a bound for i)
2) Remove identical completion items from the completions list (its a bug anyways)