This can happen if the requested pixel is not in the current clipping region. This can also happen on devices that do not support the Windows API functions GetPixel() and SetPixel(). You should check the return value from the GetDeviceCaps() to check to see if a device supports raster operations before using these functions. A printer and/or metafile canvas is a likley candidate for a device that the pixels property may fail on.

Example:
if (GetDeviceCaps(Printer.Canvas.Handle, RASTERCAPS) 
  and RC_NONE) = RC_NONE then
NoRasterOperations := True;