Clipping over sliding, definitely. It does become a bit murky, especially when people set up their charges so that they will clip (E.g. raise Zombies 1" from corner on unit, charge in, only 2 enemies in contact, so Zombies that should be annihilated survive instead), but I've had some really lame experiences after sliding. One opponent cheerfully suggested that I should slide my unit over after a charge, I thought "Why not?", and next turn he flank-charged my unit, which would otherwise have not been in range. Grrr.
Worst cheating I've seen was a guy who invented a Dwarf rune of extra leadership to help beat a friend (and in doing so claim the top spot, which should have been mine). Worst I've experienced was a Daemon player saying his Bloodthirster was S7, when he didn't have the bonus S, and so he killed my Lord turn 6, who would have otherwise survived.