If you have a smaller hobby grade quadcopter like the Heliguy Viper or one of the Hubsan X4 models (H107C, H107D, H107J) or similar.
You may not know that you can replace the motors in the other arms by simply reversing the wires so the motor spins the other way. Just solder the 'wrong' motor into the board 'plus to minus' and 'minus to plus'. This may reduce the life of the motor as the commutator should wear less in one direction. Interestingly a set of the cheap eBay replacements I bought didn't had identical insides for both motor directions..
This can be a handy tip when you have two spares and the next one to fail is one that you don't have the right motor for.
WARNING - this works on the models I fly and use that use this small 7x20mm 'can' type of DC motor. Use this tip with caution in case the motors on your model will not support this. If in doubt - don't do it.
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Happy flying!