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I get this error using BitMinter Client v1.3.2:

Work rejected. Server says: Stale or alien proof of work

about every 15 - 45 minutes, just mining on my GPU.

What is happening? What does it mean to have a proof of work rejected? How can I prevent it?

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I have no idea what alien proof of work is. Perhaps somebody should ask him on twitter. – Nick ODell Apr 22 '13 at 19:24
up vote 4 down vote accepted

There will always be some stale work that is rejected by the server.

When there is a new block on the network (from any pool) we need to switch to building on top of this new block, at the new height. The server sends out new work data to all workers. But some of them are sending in work results at that moment, and it gets rejected because it is based on the old work data.

I would consider a reject ratio of 0.5% or lower to be normal and acceptable.

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