Below is a list of the bugs that are fixed by patch 1 for Tcl 7.4
(Tcl 7.4p1):

7/19/95 (bug fix) Modified Tcl_DeleteCommand to delete the hash table
entry for the command before invoking its callback.  This is needed in
order to deal with reentrancy.  (core dumps could happen in "rename"
commands under some circumstances otherwise)

7/22/95 (bug fix) "exec" wasn't reaping processes correctly after
certain errors (e.g. if the name of the executable was bogus, as
in "exec foobar").

7/27/95 (bug fix) Makefile.in wasn't using the LIBS variable provided
by the "configure" script.  This caused problems on some SCO systems.

7/27/95 (bug fix) The version of strtod in fixstrtod.c didn't properly
handle the case where endPtr == NULL.