Re: A new(?) warning of erase-buffer, which was not seen before.

[Richard Stallman]

    In the old emacs version(s), it used to be that erase-buffer intentionally
    discarded undo information if I recall correctly. The buffer in question, 
namely
    "*sim*",  contained
    a voluminous output of a running simulator program and I simply want to
    throw away the contents from time to time. And thus, undo is not necessary
    for this buffer.

If you don't want it to collect undo information, do
M-x buffer-disable-undo, or (setq buffer-undo-list t).