Two lines, one has three spaces, one has two spaces then a tab - dedent doesn't take the two spaces away...

Python 2.7.5 (default, Nov 20 2015, 02:00:19) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-4)] on linux2
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>>> from textwrap import dedent
>>> dedent("   some\n  \tthing")
'   some\n  \tthing'
>>> dedent("   some\n   \tthing")
'some\n\tthing'

Looks very similar to issue 21827, but having upgraded to a newer build of 2.7 (Nov 2015, as above), I still see the same behaviour.  I assume the November build would have a fix made in October, but I don't know how the process actually works...