An incorrect operation safety lock device for a sliding door or window (1) comprises: an intermediate block (11) operatively connected to an operating rod (7) at the point of connection between the rod (7) and a control handle (4); a contact element (12) slidably connected to the intermediate block (11) and projecting outwards from the front of a vertical upright (6) of the sash (2) the contact element (12) has a contact head (13) which, in use, abuts against a vertical upright (10) of the fixed frame (3) when the sash (2) is in the closed position, so as to allow the contact element (12) to withdraw towards the intermediate block (11); an elastic member (14) interposed between the contact element (12) and the intermediate block (11) for keeping the contact element (12) in a position where it protrudes from the intermediate block (11) when the sash (2) is in an open position; a plate (15) for locking the rod (7), housed in the intermediate block (11), associated with the contact element (12) and movable with the selfsame element (12) from a first operating position for locking the rod (7), where the plate (15) is received in a recess (16) formed in a counter plate (5) by which the handle (4) is fastened to the sash (2), and a second operating position for sliding the rod (7), where the plate (15) is away from the recess (16) in the counter plate (5) and which corresponds to the sash (2) closed position. [ Figure 2 ]