What would make hg diff see a txt file as binary?

[Harry Putnam]

Setup: Debian testing (jessie)
Mercurial: Mercurial Distributed SCM (version 3.0.2)

Ran into something I had not seen before.

Running `hg diff' on a plain text file with some changes produced this
output:

   > hg diff common/0finaActy-snp 
     diff -r 06e8b77a6271 common/0finaActy-snp
     Binary file common/0finaActy-snp has changed

Just to make sure I quickly browsed the 3490 line file but did not
notice anything but plain text.

The regular linux `diff' command sees nothing but txt.  

Checking with the `file' command I see:
  > file common/0finaActy-snp 
  common/0finaActy-snp: ASCII text, with CRLF, LF line terminators, 
  with escape sequences

Perhaps its the `escape sequences' causing the miss-reading?