File names in OS X appearing with question marks

Question

I have a number of files on my Mac whose names suddenly started containing question marks.

When I examine them in Windows, they look fine. Same if I fire up the Ruby interactive shell (irb) and examine them in UTF8 mode.

Any ideas on how to get rid of the question marks?

Update 1: here's a screen capture of Finder with some file names with question marks.

Update 2: An example of a bad file name is: Mac? ?users.Output of locale is:

LANG=
LC_COLLATE="C"
LC_CTYPE="UTF-8"
LC_MESSAGES="C"
LC_MONETARY="C"
LC_NUMERIC="C"
LC_TIME="C"
LC_ALL=

Examining the above file name in ruby interactive shell with filename.bytes{|c| puts c} yields:

77
97
99
226
128
173
32
226
128
172
117
115
101
114
115