System V Release 4 ...

[Charles Hedrick]

There have been a series of such problems.  Several of them involve
setting environment variables or path to something unexpected, so that
you end up with the user's version of the programs.  But apparently
there are worse problems than that.  I can't tell you because I'm not
interested enough in these things to remember them.  I think the
problem is that the shell isn't just a programming language.  It
supplies lots of builtin mechanisms.  They can be used to get a
different effect from the one you had in mind.  It's not any one bug
that has people worried, but the fact that there have been a seemingly
endless stream of them.  It makes people sceptical of any claims that
the last problem has been fixed.