[tex-k] \showthe followed with undefined control sequence misbehaves

[Karl Berry]

    If an undefined command replaced itself (after the error message) with
    \relax

That sounds sensible in principle, but I feel quite sure that Knuth
would never consider such a change at this point. Changing the behavior
of expansion in any way, even with an error involved, would surely cause
problems somewhere, somehow. People have nefariously assumed pretty much
everything that can be assumed in the last 40 years :). --thanks, karl.