I have not. Are you shuffling the puzzle by applying slides to the board at random? One issue I hit sometimes is I'd have everything solved except for a 2x2 grid that seemed to have an impossible swap to make.
No, the board is randomized by swapping random tiles. I'm pretty sure now that all puzzles are solvable, though mid-board spaces do add a little extra mental barrier. I've run into 2x2 grids that I can't get but then a friend solved them very quickly, so I think there are just little niches our brains end up in that we can't shake out of. I might be wrong though!