My canapy has opened twice on my tip-up RV6.
The first time I simply forgot to completely pull it down & lock it after a long taxi in the summer heat. That was just complete stupidity on my part, but the canapy just gradually opened up higher & higher as I climbed out & speed increased...I had to slow down a bit in the climb, but was able to pull it down and latch it (really no big deal).
The second time it happened I was in a dive at about 215 mph when all of a sudden...POW...it popped open violently with a resulting downward pitching reaction (if I had been on the deck doing a high speed pass at this speed I would have ate the ground before I could have reacted in time...this scared the hell out of me I can tell you that!!). The canapy lifted up and stabilized at about 18" open at this speed, thus the reason for the extreme pitching down moment. I had to chop the throttle to idle & pull the plane back nose up till I approached stall speed, and even then it took all the strength I had to pull the canapy down & get it latched. Upon further investigation after I landed (I landed immediately after this happened) I discovered that I had latched the upper arm but apparently had not completely locked in the lower safety bar. With vibration in flight the lower bar released its lock on the bottom supports of the canapy leaving only the upper to hold it closed. With the added uplift in pressure at the 215 mph dive the upper latch pulled itself open by flexing the rear support channel and plexiglass itself resulting in a fairly deep scratch in the roll bar in the process as the handle dragged across it (luckily the plexiglass canapy itself didn't fracture and break, which I am amazed didn't happen!).
Please note that if you ever forget to latch the lower safety bar, or it ever comes loose in flight that the upper latch will NOT necessarily keep the canapy secure (especially at high speed in a dive), and could be VERY dangerous down low at high speed. This incident scared me so much that I am now installing a special plate on the roll bar that the upper latch has to lock into so that th canapy frame can't flex and disengage from...I suggest that others who have a tip-up canapy do the same.
From this incident I have concluded that leaving the canapy completely unlatched and taking off is not a dangerous safety concern. Also, if the lower safety bar is securely latched, but the upper isn't latched at all it too is not a problem. Only latching the upper one but NOT the lower one is the real danger. The other conclusion I have made from all of this is that building the plane with a slider canapy is a far better & SAFER way to go for any RV6 or RV7 in my opinion!!!