I will assume you're having an issue with a chain saw. If that's the case the chain can be getting tossed due to a bad chain, a bad bar, or a bad sprocket. In most cases the bar and chain tend to be the problem. If take the chain off you can use a straight edge to see any bends in the chain and bar. Also, you can check the sprocket when you remove the chain and bar to make sure it is not worn down.
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