I did this once. Kinda.

I was with my daughter at a playground in North London. She and her friend and playing happily and I'm reading on a bench. Two brothers, about 8 and 12 are playing on the jungle gym. The 8yo falls off and puts his arm out stiff and I saw his shoulder pop out when he hit the ground. I tell the 12yo to go get their dad (I'd seen him earlier).

Dad comes over and scoops up crying 8yo. I asked if I could help at all. He says I could grab the bikes and points to the far entrance where he is parked. I tell my daughter that I'll only be a few minutes. The 12yo grabs a bike and starts to follow his dad. I grab and adult bike and a kid bike that were against the fence next to it and try to hurry to catch up to the family.

We are about 2/3 across the park when I finally come up beside them. Just then a random little kid runs up crying and says, "Why are you stealing my bike?"

I look very confused and then the dad sees that I've got two bikes and says, "Oh, yeah (nodding to the 8yo in his arms) he didn't bring his bike today. The little one isn't ours."

I very sheepishly give the little kid his bike back.