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 a > a
hg add a
hg commit -m test -d '0 0'

# Default operation
hg clone . ../b
cd ../b
cat a
hg verify

# No update
hg clone -U . ../c
cd ../c
cat a
hg verify

# Default destination
mkdir ../d
cd ../d
hg clone ../a
cd a
hg cat a