Stupid mistakes almost make one turn against parcel firm (4-3)

'stupid mistakes' is the definition.

'almost make one turn against parcel firm' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'make one turn' becomes 'gooff' (to go off can mean to make a turn in direction).
'against' is a charade indicator (letters next to each other).
'parcel firm' becomes 'ups'.
'gooff' with its last letter taken away is 'goof'.
'goof'+'ups'='GOOF-UPS'