Re: "Attempt to modify read-only object" error with set-frame-configuration

[Richard Stallman]

    GDB shows that old_alist_elt's value is 18412069 decimal or 118f225
    hex, which indeed seems to be in pure space.  The value of
    f->param_alist includes `(visibility)', and I verified by stepping
    into Fassq that it finds this element of the alist, and then this last
    line of Fassq:

The bug is making that cons cell pure in the first place.
These cells are all meant to be setcdr'd.
So if you (someone) can find where that cons cell is made,
you will find the bug.