Some interesting comments on here...
If you claim to have lost your passport and get another, the original is blocked for ever and if someone tries to use it then a long chat with border personnel will take place.
Any lost or stolen passports normally require a police report.
A replacement for water damage would work once, maybe twice then questions will be asked.
And as for ripping a page out!!! You will be found out eventually and again an interesting chat will take place. Remember every time you present your passport a record is kept so it wouldn't take long for the missing stamps to be worked out.
Mislaid mine once and panicked, found it a few days later. Problem being I was in Budapest and had already been to the embassy so I couldn't continue to use the old passport, several pages and the cover have a corner cut off before receiving your new passport. I was allowed to keep the mislaid passport.