(This post is constantly updated)
There seems to be an issue with the geth client for ethereum nodes at the moment.
Some bad guys want to harm ethereum. They exploited a bug in geth client which leads to fail all geth client based nodes.
The transaction payload that lead nodes to stop working can be found here:
https://etherchain.org/tx/0x5c19695f50a30abbadfeef201d695d9c95c254534019e2f6a7a590e9ef246e82
What does the payload means in ASCII? 'Fahrt nach Hause' - that means Go home in english and possibly refers to Devcon 2 participators which starts September 19th 2016 (Monday).
You can see the issue reported on github here:
https://github.com/ethereum/go-ethereum/issues/3002
As far as we know only geth and not parity is affected. Thanks to different client the ethereum network is still running.
http://ethermine.org (Ethermine mining pool) has switched to parity client, others will follow.
Gustav Simonsson from the Ethereum foundation wrote on reddit that the devs are working on the issue and a post will be made at https://blog.ethereum.org the next hours.
EDIT2: Post can be found here https://blog.ethereum.org/2016/09/18/security-alert-geth-nodes-crash-due-memory-bug/
EDIT5:
Twitter post from ETH Foundation:
https://twitter.com/ethereumproject/status/777621041724157953
- ethereumproject
Kraken stopped ETH Funding:
https://twitter.com/krakenfx/status/777623594985590784
- krakenfx
Poloniex stopped ETH Funding:
https://twitter.com/Poloniex/status/777145303753494528
- Poloniex
At the moment the network seems to be safe, hashrate is growing while people are updating to parity.
See the current stats @ https://etherchain.org or https://ethstats.net
Price fell for a moment and is now relatively stable a little over 0.02 BTC. (Price ticker here https://cryptowat.ch/poloniex/ethbtc/1h)
Interesting part is: ETC is even more at risk. Anyone could run the same contract and transaction on the ETC chain. Bad part is that ETC has nearly 100% geth clients which would lead to network halt. If someone has bad intentions, that person could exploit the bug and run a 51% attack on ETC (it has way less hashrate). The trading on ETC (Kraken) is totally irrational though: https://cryptowat.ch/kraken/etcusd/1h
As we observe ETH and ETC have a strong negative correlation in their prices. However, ETC is even more at risk and price should fall even more. Shorting ETC would be a good idea now. Whatever.
EDIT1 :
The adress of the contract that exploits ('from Shanghai with love'):
https://etherchain.org/account/0xb284e6a25d0972f9a92fec45d2075067db2d49b0
Found these 2 transactions:
https://etherchain.org/tx/0x0e402e44782c627edb6f2e25f5301cc505ccc3605674bc8b5c124fbe567ff4c2
Payload: 'Weltreise' ('world journey' in english)
https://etherchain.org/tx/0x0e402e44782c627edb6f2e25f5301cc505ccc3605674bc8b5c124fbe567ff4c2
(not sure about the payload)
be careful when using 
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Stay tuned. I will update this post as soon anything happens!