A failed enchantment to make a flying carpet creates ... a carpet that keeps your feet warm when placed over a cold floor.

A failed enchantment to make a ring of protection succeeds ... at making a ring that protects the little bit of your finger it covers.

A failed attempt at a potion of alertness gives you ... a cup of coffee.

A failed try at making a bag of holding gives you ... a bag that will hold a volume equal to its external measurements.

What else?