Unfortunately, wire strikes are quite common. Balloon crews carry OS maps overprinted with "half mil" information for navigation so pylons are marked. Crews are also trained to look left and right to spot lines of poles and so avoid gift wrapping the National Grid. Sadly, even the best pilots have been known to decorate power lines with half an acre of bright nylon.
I will add a good story when I get the time to write it properly.
Brian.