Upvote:1
I didn't once, and I got told off.
I was on Greyhound back into Canada, and they just cruised on through, easily, without handing in anything.
Then the next time I was entering the US, he opened to see my I-94 slip. He got quite vocal about how I was meant to return this, I apologised and he just shrugged and shoved it in a drawer with many, many more.
The next time I crossed the border, I insisted that I give mine to the Canadian border guy, who seemed a bit bemused, but took it anyway.
Short version: I did this, got a telling off, that was the extent of it.