The answer is YES! Receiving a PPR is just a step in the whole process. There can be a number of things, highly unlikely, but can happen. A few examples are:
1. An applicant misrepresented and it was found by CIC, even if they have issued a PPR, they can refuse to grant the COPR.
2. In a unlikely event of the applicant being involved with a legal issue within his home country which calls for impounding the passport, the same will be notified to CIC and this can bring in trouble.
3. Even when the COPR is issued, if an applicant is charged with a criminal activity, his PR can be terminated. This can be within, or outside Canada.
4. If CIC finds that the applicants family member is inadmissible, even if they have issued the PPR, they can refuse COPR.
There are numerous possibilities, but the above mentioned 4 are the big ones. Bur even as a permanent resident in Canada, an applicant has to be careful to comply with the laws or else his PR can be cancelled and he can be deported.