I put a newly received passport in the washing machine. The covers separated from the first and last pages, no other damage. I glued them back together, and travelled with the passport for some years, until my glue started to fail. An immigration agent asked what had happened, I told my story, he explained that regluing the passport could be considered "tampering". The next agent also noticed, and at that point I had the passport replaced.

Point is that I did not see the cover as an integral part of the legal passport, but I was wrong.