And why do I get this error?
kat:~/Pike/svn-test-7.7/src> svn log -r26702 program.c svn: REPORT request failed on '/repos/pike/!svn/bc/26702/Pike/7.7/src/program.c' svn: '/repos/pike/!svn/bc/26702/Pike/7.7/src/program.c' path not found
The reason is that the name "program.c" is expanded to the path "Pike/7.7/src/program.c", and in version 26702 that path does not exist. The path that you _want_ to log in that version is "Pike/7.5/src/program.c". If you have svn 1.4, you can use a "peg revision" to say "the path which I want to log in -r26702 is the one which corresponds to the one which is called Pike/7.7/src/program.c (abbreviated as program.c) _now_".
See http://subversion.tigris.org/issues/show_bug.cgi?id=2460