pymongo/helpers.py
     57     elif not isinstance(index_list, list):
     58         raise TypeError("must use a list of (key, direction) pairs, "
     59                         "not: %r" % index_list)

If index_list is itself a tuple, which can happen if I forgot to put the sort pairs in a list, this string interpolation fails, and I don't get the intended error message.

Line 59 should be :
"not: %r" % (index_list,))