Glider canopies occasionally come loose. Rear opening canopies promptly depart the airframe. You may not notice in flight that a front opening canopy was not completely latched (don't ask).
Side opening canopies can be kept down with a bit of rudder.
Sadly glider and tow pilots have been killed when the glider pilot focuses on closing a side opening canopy instead of flying the airplane.
A towpilot was killed last year in the US when the glider got high on tow while the glider pilot was trying to close the canopy
