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...

pulling from ../a
searching for changes
warning: pulling from an unrelated repository!
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
(run 'hg update' to get a working copy)
changeset:   1:9a79c33a9db3
tag:         tip
user:        a
date:        Thu Jan  1 00:00:00 1970 +0000
summary:     a

changeset:   0:01f8062b2de5
user:        b
date:        Thu Jan  1 00:00:00 1970 +0000
summary:     b