bug that vanishes after a load (SF bug 2014936)

I was looking at SF bug 2014936. Here is something that might be of a
more general nature. Using gcl, here is the bug:

 (%i1) at(f(a,b),a=x,b=y);
 (%o1) f(x,b)

Maxima ignores the third argument to 'at.'

Let's load "comm2.lisp" (5.15.0).  After that, we get a Lisp error
for the same input. Why does the bug disappear after the load?

 (%i2) load("comm2.lisp")$

 (%i3) at(f(a,b),a=x,b=y);
Maxima encountered a Lisp error:
 Error in $AT [or a callee]: $AT [or a callee] requires less than three
 arguments.

I think this bug might be specific to gcl; at least I think I don't
get it with SBCL.

Barton