I recently did a flyer which was printed digitally and noticed a couple of instances where the bounding box of an image with transparency had effected the colour background underneath.
The flyer was printed from pdf, transparency flattener was set to high. The images were tiff with clipping paths. They were stock photos but I applied a path cos they hadn't been cut ou too well.
There was no indication of a problem either on screen or in the pdf.
My guess is that:
a)itís a drawback of digital printing (cos the same thing happens on bog standard Lasers, ...