There was a member on this board who was under the impression that his PR was protected because he was working for a Canadian company overseas. It turned out that the company did not meet the requirements to count as a Canadian company overseas and his PR was not protected. After he came back to Canada and tried to renew his PR card, he was called in for an interview and given a serious talk about it but in the end they let him keep his PR because of his wife and kids having PR and living in Canada. Otherwise, his wife would just have sponsored him again anyway. This is the only specific case I know of but at
http://www.cic.gc.ca/english/resources/manuals/op/op10-eng.pdf you can read the operational manual for PR residency requirements. It goes into H&C reasons in section 5.4, page 9. If they give the option of applying with H&C reasons, I am sure there are some cases where they approve them.