Test a step in Lava and Aqua
The lava spreads with each move. Take a step, look at the new lava edge, and press Z if the way to the exit has narrowed. The undo pulls the lava back as well, so the grid is exactly as it was.
Go back several moves
Each further press of Z goes back one more move. Stop when the route opens again and try a different direction from that square. R resets the level completely.
Correct a cell in Nonogram Square
Nonogram Square lists Z for undo and Y for redo. Use them when a filled cell breaks a row or column clue. Marking certain blanks with an X makes it easier to spot which fill caused the clash.
Apple Worm restarts the level
Apple Worm has no undo. Its round-arrow button sends the worm back to the start of the level. Before turning the head, count how many segments have to follow it round the corner.
Choose between undo and restart
Undo keeps the part of a plan that worked; a restart suits a plan that went wrong from the first move. Jumping Clones resets with R, Blumgi Slime has a restart button and Poki lists no recovery key for Rotate.