Square brackets are most commonly used around the word 'sic' (from the Latin 'sicut', meaning 'just as'), to explain the status of an apparent mistake. Generally, sic means the foregoing mistake (or apparent mistake) was made by the writer/speaker I am quoting; I am but the faithful messenger; in fact I never get anything wrong myself. Book reviewers in particular adore to use sic. It makes them feel terrific, because what it means is that they've spotted this apparent mistake, thank you, so there is no point in writing in. However, there are distinctions within sic: it can signify two different things:
1) This isn't a mistake, actually, it just looks like one to the casual eye.
2) Tee hee, what a dreadful error! But it would be dishonest of me to correct it.
~ From 'Eats, Shoots and Leaves' by Lynne Truss
1) I am grateful to Mrs Bollock sic
for the following examples.
2) "Please send a copy of The Time's sic
," he wrote.
~ Again, from 'Eats, Shoots and Leaves' by Lynne Truss