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I haven't got that far in the game yet, so I am sure there is stuff I haven't seen yet that makes the game harder.
For now my idea of impossible would be a key placed in a separate board, and surrounded by liquorice swirls. The position of the key means it can only be affected by the vertical blast of a striped candy. Oh, and obviously you only have fifteen moves.
Level design mistake recently. There were ingredients required, and the only way to get them to fall was through a dispenser. That single dispenser was blocked by Candy Cane. No cherries could fall, even with 200 moves. Example of a glitch maybe, but was level designer error that slipped the net and was on Win 10 a while.
Difficulty can make a level "virtually impossible" if an exceedingly lucky board is needed of about 500 attempts.....most players will mark it impossible as there is no fun and all frustration with no inventory. I have inventory and can move on myself. To take a key surrounded by licorice, or any other blockers involved like Licorice Fence is by no means impossible, with inventory. A single lollipop hammer or a CB x 2 combo takes the key in @Alienscar's level.