The BadMatch error can occur if the specified rectangle goes off the edge of 
the screen. If you don't want to catch the error and deal with it, you can take
the following steps to avoid the error:

1) Make a pixmap the same size as the rectangle you want to capture.
2) Clear the pixmap to background using XFillRectangle.
3) Use XCopyArea to copy the window to the pixmap.
	[Whoa! this answer is currently under reexamination.]
4) If you get a NoExpose event, the copy was clean. Use XGetImage to grab the
image from the pixmap.
5) If you get one or more GraphicsExpose events, the copy wasn't clean, and 
the x/y/width/height members of the GraphicsExpose event structures tell you 
the parts of the pixmap which aren't good.
6) Get rid of the pixmap; it probably takes a lot of memory.

[10/92; thanks to Oliver Jones (oj@pictel.com)]