I like to print on pre-printed stationery using the Preview.app and a duplex-capable HP Color Laserjet 4700 (PostScript) printer. The print dialog handles one and two-paged documents differently: the paper needs to be placed differently into the tray if the document contains one page versus when it contains two pages. This is not obvious when printing on plain paper but becomes obvious when front and reverse side of sheets are marked. Otherwise the first page would end up on the reverse side of the first sheet.
I believe the problem is caused by the printer driver setting duplex printing to false (using the PostScript setpagedevice operator) when emitting a single-page document versus keeping it set to true when emitting multi-page documents. All this despite that duplex printing is always specified in the printer dialog. When printing a single-sided document, duplex=true and duplex=false seem to make a difference with respect which side of a sheet gets printed on.
It would be also helpful if others could confirm the problem actually exists. I suspect this problem is not limited to specific printers. I'm on OS X 10.6 and I checked two different HP printers.