A family of Microsoft word processing software products for creating web, email, and print documents.
Each character can have a different language parameter applied (or no proofing). If part of the word is formatted with one language and the rest (or a single character) with another, then you are likely to get part or all of the word flagged as incorrectly spelled.
For example type the word 'Setting', then change the language of the second 't' to another language (commonly there may be transitions between English US and English UK - particularly where material is copied from the internet).
The first part of the word 'Set' is a valid word and so will be accepted. The second half of the word 'ting' is treated as two words 't' and 'ing', one with the changed language the other with the original language. 't' is not flagged as a single letter, but 'ing' is not a valid word and is thus flagged as incorrect.