I have seen an engine backfire and begin to run backward in flight... for it to happen the engine has to be WAY out of tune, compensating for an air leak or some other problem that really needs correction.

It is rare, but it can happen.

If your engine starts backward... rapidly cycle the throttle a couple of times and it will usually kick itself to running forward.

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there is one other time it can kick itself into running backward in air... be doing a rapid throttle transition just as it runs out of fuel... it can run backward for a few revolutions before it dies.