This is discussed in ENF 4, which refers to port of entry examinations. While the PR card proves permanent residency as a general rule, it is not the only way for a BSO to determine that you are a permanent resident - basically they can find you in the computer. Worst case, they send you to the secondary inspection line and they look you up in the computer. Once they have established you are a PR, they must allow you to enter (even if you are in breach of the PR obligation.)