[Ethereum] Bad block, gas used error, Synchronisation failed

If you are trying to sync with testnet, there seems to not be many peers running on this testnet. I got the same errors, but just restarted geth and eventually the blockchain was synced.

I've not encountered this problem on mainnet. Please reply / edit your question if you did encounter this issue on mainnet and we can try to work through this problem.

You're hitting an errUnknownPeer error.

From the code in downloader.go:

// Retrieve the origin peer and initiate the downloading process
p := d.peers.Peer(id)
if p == nil {
    return errUnknownPeer
}

Here we're trying to use a previously known peer with an id of id, but which no longer appears in the list of known peers. (So our look-up in the current list of peers returns nil.)

There's nothing in the code to explicity define "unhealthy" with regards to peers - it only appears in the error string - but from the context it means either:

• the peer is no longer online, or
• the peer is online but unable to communicate.

Both of which means the peer can no longer be discovered.

I can't see anything in the code that would log anything more useful, but you could try setting a higher verbosity level in the console if it happens repeatedly. (Using debug.verbosity(x), where x is the logging level.)