I have this error from 5 days ago on my 2 servers
WARNING: abnormally high number of blocks generated, 48 blocks received in the last 4 hours (24 expected)
What is this error ? I need to reindex the chain ?
Luke-Jr commented on this error here: https://www.reddit.com/r/Bitcoin/comments/3ydwg2/warning_abnormally_high_number_of_blocks/#oo
What this means:
48 blocks were found within the last 4 hours. The average is "supposed" to be 1 block every 10 minutes, or 24 blocks over a 4 hour window. Normally, however, blocks are found at random intervals, and quite often faster than every 10 minutes due to miners continually upgrading or expanding their hardware. In this case, the average has reached as low as 5 minutes per block, which triggers the warning.
If the network hashrate was not increasing, this event should occur only once every 50 years. To happen on average, persistently, the network would need to double its hashrate within 1 week, and even then the warning would only last part of that 1 week. So this is a pretty strange thing to happen when Bitcoin is only 6 years old - but not impossible either.
Update: During the 4 hours after this posting, block average seems to have been normal, so I am thinking it is probably just an anomaly. (Of course, I can't prove there isn't a new miner that has just gone dark or mining a forked chain either, so continue to monitor and make your own decisions as to risk.)
Why is this a warning?
It's possible that a new mining chip has just been put online that can hash much faster than the rest of the network, and that miner is now near-doubling the network hashrate or worse. They could have over 51%, and might be performing an attack we can't know about yet. So you may wish to wait for more blocks than usual before considering high-value transactions confirmed, but unless this short block average continues on for another few hours, this risk seems unlikely IMO.
Has the blockchain forked?
No, this warning does not indicate that.
Will the warning go away on its own?
Bitcoin Core will continue re-issuing the warning every day until the condition (>=2x more blocks) ceases. When it stops issuing the warning, however, the message will remain in the status bar (or RPC "errors") until the node is restarted.
Edit: Note that although the gist of the explanation above seems to be correct, the exact number of "once every 50 years" seems to be incorrect (bug in bitcoind code) and is probably more like once every 4 years. Details are in the replies to the above Reddit post.