CVS is a wretched hive of scum and villainy. Lifting a CVS repository into a clean revision history is difficult and has rebarbative edge cases. If you are reading this, you have probably tripped over one of them. Here is what to do about it.
First, reduce the test case to a minimal set of CVS files that will reproduce the misbehavior. The cvsconvert wrapper script should be useful for getting a concise summary of errors visible at tagged locations and branches.
There is a tool called 'cvsreduce" in the cvs-fast-export distribution. It makes a skeletonized copy of a CVS repository, dropping out all the content but leaving the metadata in place. Revision comments are replaced with their MD5 hashes, very short but still unique.
The skeletonization process has the additional benefit of removing any sort of data that might be sensitive; only filenames, revision dates and committer IDs are left in place.
A conversion attempt on the skeletonized repo should raise the same errors as the original did (except for ignorable ones involving CVS keyword expansion, which will all go away when the file is skeletonized). If it does not, try skeletonizing with -t instead to preserve non-sticky tags
Now try to make the skeletonized repository as small as possible by removing swathes of files from it, checking each time to make sure the error continues to reproduce. It is best if you can reduce the fileset to a single file or pair of files.
You should find you can simplify the directory structure by moving files from subdirectories to the root, doing file renames to avoid mame collisions. Neither moves nor renames should change the errors reported except in the obvious way of changing pathnames in the messages. Again, if the errors do change in any other way this is interesting and should be reported.
At the end of this process you should have a handful of skeletonized master files.
Make a tarball including the CVS masters. Make a session transcript showing the error relicating on them. Mail these things to the maintainer with any other information you think might be relevant.
Wrestling with CVS repository malformations is ugly, hard work.
If you are requesting help on behalf of an open-source software project, you will get help for free as the maintainer’s schedule permits.
The maintainer is available on a consulting basis to all others and will expect to be paid for his pain.