The Office for Standards in Education warned inspectors in its Update newsletter this month that it had received a number of reports of teams forgetting sensitive papers or leaving them where they could be read by teachers and pupils.
Many of the documents were left in the rooms allocated to the inspectors, but others had been dropped elsewhere.
"In one case, these were briefing papers containing comments of a personal nature about members of the school staff and critical remarks about the grounds," the newsletter said. "The discovery of these papers caused distress to the headteacher, staff and governing body. Inspectors should ensure that no documentation is left behind at the end of inspections, and that recorded comments do not include references of a personal nature."
Geoffrey Panzer, director of Open Book Inspections, said that it was highly unusual for inspectors to lose their notes but that inevitably it would happen occasionally at some of the 4,000 schools that are checked each year.
Inspectors tend to refer to teachers on their evidence forms using code names, and then destroy the code key at the end of the visit.
Mr Panzer said: "If someone found one of the evidence forms they might be able to figure out that Teacher Y was really Miss Bloggs through a process of elimination because she was the only teacher teaching Year 6 maths.
"Critical comments must never be personal and can only relate to the education provision. You could not make personal comments about the way a member of the PE staff dressed, for example, but you could say that it was inappropriate for teaching PE."
The Update newsletter also warned inspectors against giving advice on administering medicine. An inspector was reported to have caused unnecessary distress to a boy's family and school by insisting that he receive medication for cystic fibrosis in a different way.