I just typed draft instead of drafts when uploading one of my changes to Gerrit. Thus the ref refs/draft/master/T44180 was created. Because I am a member of the mediawiki group, the system allows me to create a ref anywhere under refs/ (with some exceptions of course). Clearly in this case I did not mean to.
It seems, however, that I'm not the only one to have made this mistake. This has apparently happened a few other times:
$ git ls-remote origin | grep -vPe '^.{41}(HEAD$|refs/((cache-automerge|heads|tags|remotes/origin)/|(changes/\d{2}/[1-9]\d*/[1-9]\d*|meta/config|notes/(commits|review))$))' adfcc85ab77c30c6928994bb75a1423b7b522f34 refs/draft/master/2012/jobqueue-redis 60a8c73d127f229eb47d0424a95877980cd6ace1 refs/draft/master/2012/next-job-abstract 79a4cc0fa268f04854939d72281c4b2b9e152fac refs/draft/master/2013/deferred-update-cancel ca00e2c7d394d1628574226873613f5e812e1aa4 refs/draft/master/T44180 892329cfba8569586ee7bb81d7964a60ca1a12a2 refs/draft/master/bug45355 46dcd1d5cb1122ec0965643a0ea5572bc0e4cbe3 refs/sandbox/adamw/refactor_protection ecc958d7f7ac697043334a113a68e2ac078c4d13 refs/wmf/1.22wmf18 0b6a90ec591bfb2f2a8f2ed05fbf8e901f8d367a refs/wmf/1.22wmf19
I think all of the refs listed above are no longer necessary and can be safely deleted.