Been there, done that
In our case (many years ago but it lives on in my mind) the whole of the downwind side came free - and a flapping polytunnel cover in 80mph wind packs a powerful punch. We hadn't put the little bits of wood that attach the polythene to the base rail on the right way round. OH with flu and in his jim-jams had to swing from the crop bars and cut the polythene right down the middle of the roof so of course new cover required. I'm definitely not laughing at you sotb - I found our adventure quite scary, especially when I tried to hold on to a corner to stop it flapping
Since then we have had several holes and a tear - courtesy of a 4 horned Jacob who cut a neat triangle in the cover so he and his mates could step inside and demolish the brassicas. The very best way to repair torn polythene is with the widest repair stuff sold by the tunnel manufacturers. It's clear tape and sticks really well - we have never had a repeat repair in the same place. It is expensive but duck tape doesn't last long in the wet.
The trick to not getting tears, is to put the cover on extremely tightly, on a warm day, so it's soft and pliable. The ends are the most difficult bits with pleats etc, but it's worth fiddling with to get it right.