Encountered this using cygwin's rcs 5.8.1-1 and 5.8.2-1.
Earlier version 5.7-11 does not have this bug
(so I've been specifically reverting to rcs 5.7-11
every time I run setup.exe).

Checking in a text file of size >= 256k
corrupts the rcs file, irretrievably losing most of the contents
and making future operations impossible.

The attached script demonstrates.
Run it in an empty directory (or a directory containing only the script).

Output of cygcheck -s -v -r also attached as cygcheck.out.

-- 
Don Hatch
hatch@plunk.org
http://www.plunk.org/~hatch/