This is probably a design consideration rather than a bug, but it sure bugs me. :)

Cancelling a SuperFly Progressive Refinement render part way through a sample gives a final render with some completed buckets from the previous sample and some from the interrupted sample. This can give very obvious bucket boundary artifacts (especially in PNGs, not quite so obvious in JPGs), and the whole render is useless. The final render image really ought not to be updated with the partially-complete sample.

It's possible to avoid this by keeping an eye on the bucket count and cancelling the render at the exact point at which it starts a new sample - preferably just before rather than just after, since the new sample buckets will be dead centre of the render. The scene can be framed such that any incomplete buckets at the outside can be cropped.

This is on Pro 11, Mac OS X version.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)