Well we all know the people who 'assess' our documents aren't always following the rules so I was thinking that if a canceled passport was 'scanned' or checked you would be in for a big hassle explaining things to whatever napoleon was on duty.

Second if you did inadvertently use the wrong passport to cross a border again hassles would ensue.

For simply flashing to some geek at a gate to compare to your BP or at a hotel desk sure I can't see any troubles.

Personally I'd like to have 5 or 6 'different ' passports like Jason Bourne and avoid the whole mess all together