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a) If 'tcsh' is run as non-login shell, the '$history' value has greater value than the value in '$savehist', which is bad according to tcsh documentation. b) If 'tcsh' is run as login shell, the '$savehist' does not have the keyword 'merge' inside. This may (quite unlikely, though) trim the ~/.history file.
I problem b) should be treat as high severity. If user has set the default shell to 'tcsh', the first starting and last shutdown tcsh instance is run as login shell - thus, most likely, the history file is going to be truncated.
Ok, scratch the comment #1, that is not truth. I had misconfigured local settings. But the a) case seems to be still issue, though with much lower severity.
Looking it once again and even the a) case seems to be innocent to me (the only thing what happens is that max(savehist, history) number of history entries will be saved at shutdown). Thus closing, sorry for noise.