My DS has done this a few times, little bugger
The first time wasn't really his fault, I used to take him to Nursery and explain every bruise, my mum took him and forgot, he said I threw him across the floor and he landed on his eye. 
What happened was, he locked me out of the kitchen by putting the bin against it, I panicked as the cooker was on, took and run and jumped and kind-of karate kicked the door, he was behind the bin, behind the door, I threw the door open, he landed beneath the bin...
The second time, I have no idea why he said this but he said 'my mum was furious last night because of parents evening, I got the biggest smack ever', so the Teachers took him aside asked about the smack, where it was etc, he came home, told me I had been talking to school about him, told me the story about Teacher checking him for smack, I phoned the school furious and once again, it was him!
He still does it to me, I can be taking him to bed, sometimes I shout on my 12 year old to take him as if I have to touch he him he will say 'you're throwing/punching/pulling/pushing/dragging me'....
He was getting changed tonight whilst a school friend was waiting and accused me of kicking him, thankfully, everyone could see I didn't move!
I agree it is humiliating and embarrassing but people have to act on what the DC say, some are being truthful!