The activation of the Instanbul upgrade in the Ethereum testnet has reportedly split the network into two separate chains.
The community manager of the Ethereum Foundation said on Twitter:
"Looks like there is an issue with the Ropsten Ethereum testnet after it hit the #Istanbul block. This is what testnets are for! Be aware that Ropsten will be unstable until this all plays out."
For reference, it was expected that the testnet would be upgraded on October 2 at the block 6485846. However, the actual upgrade date was September 30.
Notably, miners in the Ropsten network failed to install the latest software as like developers the early upgrade was a real surprise. Thus, the updated network and old one started existing separately.
Such a scenario was observed last October when the Constantinopole upgrade split Ropsten for several hours.
Hudson Jameson stated that the main challenge of PoW testnets is coordination of the miners. For now, the company is trying to move miners to the correct chain. He believes that two separate chains appeared just due to too slow miners rather than Istanbul bugs
The hardfork of the mainnet will be discussed during the conference call on October 4.