In this example characters between percents are parsed as for any conversion specifier but ignored and '%(a) %' is formatted to '%'. '(a)' specifies a mapping key, ' ' is a conversion flag, the second '%' is a conversion type.

>>> '%(a) d' % {'a': 123}
' 123'
>>> '%(a) %' % {'a': 123}
'%'

This behavior is explicable but looks weird and errorprone. I'm not sure about changing behavior in maintained branches (technically this may be not a bug), but I think it is worth to make this an error in the developed branch.