buffer~: can't resize now. Why???!!!!

Mac OSX latest, MaxMSP latest

I keep getting a "can't resize now" message; what are the possible reasons for this?

The offending patches...

I have a temp buffer~ that I load samples into. I then select a portion of that buffer (in [waveform~] and copy it into a different buffer using [clear], [size ...] (derived from the waveform~ selection and set in milliseconds) and then [peek~].
The first time I do this it works ok. If I then copy a differently sized segment over, I get the "can't resize now" message. The selection copies over to the buffer, but leaves a gap at the end (the extra length of the previously copied sample), or truncates it in the case of a longer selection.
Having audio on or off doesn't make any difference.
I've tried resizing the destination buffer~ using both a [prepend size] message and sending the new size to an attrui.
If i open up the subpatcher containing the destination buffer and try manually changing the value in the attrui, max freezes (and, incidentally, leaves me with with a coloured wheel cursor even if I force quit max).

{ (Edit) - I've just added a [crop x y] message to the destination buffers - that doesn't work either, and brings the "can't resize' message up.
incidentally - the crop message box that is created when selecting the item in the contextual menu only has a single $1 argument, rahter than the 2 required. }

What kinds of situations would bring up this error message? (It's not documented in the docs as far as I can see).