How does Git(Hub) handle possible collisions from short SHAs?

Question

Both Git and GitHub display short versions of SHAs -- just the first 7 characters instead of all 40 -- and both Git and GitHub support taking these short SHAs as arguments.<

Answer 1

I have a repository that has a commit with an id of 000182eacf99cde27d5916aa415921924b82972c.

git show 00018

shows the revision, but

git show 0001

prints

error: short SHA1 0001 is ambiguous.
error: short SHA1 0001 is ambiguous.
fatal: ambiguous argument '0001': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions

(If you're curious, it's a clone of the git repository for git itself; that commit is one that Linus Torvalds made in 2005.)