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 foo > b
hg add b
hg ci -m "b" -d "0 0"

chmod -w .hg

cd ..

hg clone a b
cd b
hg verify

cd ..

chmod +w a/.hg # let test clean up