OpenGL's complainyness

["Keith Lucas" <klucas@atlan-tech.com>]

OpenGL (well, Mesa in fact) has suddenly started complaining when I try and
begin primitive rendering modes: I can use arrays, I can draw the glu primitives
like cubes and so on, as soon as I try and start GL_QUADS, I get an error which
translates to "illegal operation".

Anyone any idea what causes this behaviour? I thought it might be already in a
mode, but a glEnd is an illegal operation as well. This is on entry to my draw
call back first time. It clears the window OK, and will flush the window. But I
can't draw QUADS or POINTS or anything.

Is there any way to get better diagnostics out?