Halliday & Matthiessen (1999: 478):
Intentionality can be construed in the clause as circumstances of Manner (‘according to plan’, e.g. he found the book by chance; he turned left by mistake; he turned left intentionally) and as verbal group complexes of projection or enhancement realising the Process (e.g. she intended to leave at 4; she happened to be in the neighbourhood). It may also be a factor in certain lexical contrasts within delicate process types; but we have found no evidence that intentionality is a primary variable in the semantic system of figures and its grammatical construal in transitivity. Rather, whether an example is read as construing something as intentional or non-intentional will depend on a variety of factors, such as the consciousness of the most active participant.