Apparently I need more practice taping my moving boxes! This is what I woke up to after packing some last night. 
It is an art. Make sure you press the tape down firmly across the whole strip.
that kinda tape always comes undone for me too, i prefer using this stuff: 
House pixies for sure
Oh yeah that is not good.
I was a shipper for years and Scotch heavy duty is your friend. Also, a piece perpendicular to the flap opening provides major strength. It never caught on because of cost but it's truly a game changer.
LOL