Fortuitous bit of scoring? (10)
I believe the answer is:
accidental
'fortuitous' is the definition.
(I've seen this in another clue)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for accidental that I've seen before include "Chance, inadvertent" , "indication to modify tone slightly" , "not intended by composer?" , "sign showing that flat may be got rid of?" , "Subsidiary" .)