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...

diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123
diff -r 3903775176ed a
--- a/a
+++ b/a
@@ -0,0 +1,1 @@
+foo
diff -r 3903775176ed b
--- /dev/null
+++ b/b
@@ -0,0 +1,1 @@
+123