Hedge-trimmer slices bus roof.
 

http://www.bbc.co.uk/news/uk-england-20623054

I wonder how?

Yikes! That could have been really nasty.

Wilkinson's, later Lex Wilkinson's, the parcels carrier, that collected from where I worked in the early 80's had something similar happen to one of their Ford D series vans. Except, the tractor had a bucket, not a hedge trimmer. He had pulled up to the STOP/GIVE WAY line, with the arms raised but, although the bucket was high enough for cars etc., it wasn't high enough to lear a 13/16t. box body. The truck driver had seen the tractor but, not the bucket. CRUNCH!!!!!!

I remember something similar happening at Trearddur Bay in the late 90's where a tractor driver was driving along the farm drive to join the main road. The tractor had fork lift prongs on the front which were lifted high but as the tractor came to the end of the drive a School double decker went past!

Could have been nasty.