When my son was born 16 years ago I had to have him put on my passport, so this meant a new passport for me. By the time mine expired (after 10 years) minors could get their own passports, so both my son and I got our new seperate passports.
The year before last, about 2 weeks before we were supposed to fly out I was flicking through everything making sure it was in order, when I checked my passport to make sure it was in order, i assumed his was too (as we had gotten ours together), alas I forgot, or actually I don't think I knew/realised that while mine was for 10 years, his was only for 5 years.
I realised 3 days before we flew and again it was down to chance as we were comparing the photos on the passports and the expiry year caught my eye.
I nearly dropped!! I immediately went into the PP office and had the passport in time, they were great, it was an expensive mistake to make all the same.