AmbiguousCommand is raised too soon. Right now, hg raises AmbiguousCommand as soon as it finds two commands/aliases that start with the substring it's searching for, even though it may still find a full match later on. This is a bit hard to hit on purpose, because hg checks the list of commands in whatever order is returned by table.keys(), which will change when you add an alias to a command. You should be able to hit it by adding an alias "u" to the "identify" command - not that that makes a lot of sense...

#!/bin/sh

mkdir a
cd a
hg init
echo 123 > a
hg add a
hg commit -m "a" -u a -d "0 0"

cd ..
mkdir b
cd b
hg init
echo 321 > b
hg add b
hg commit -m "b" -u b -d "0 0"

hg pull ../a
hg heads