Ethereum test network Ropsten has forked into two separate chains following the activation of system-wide upgrade Istanbul.

“It appears there are two different chains mining the Ropsten test network. There are miners mining on the old [Ropsten] chain and miners mining the new one,” explained Ethereum Foundation community manager Hudson Jameson, adding in a tweet:

“This is what testnets are for! Be aware that Ropsten will be unstable until this all plays out.”

Originally expected to activate on Oct. 2 at block height 6,485,846, Istanbul was released two days earlier than planned – on Sept. 30 at roughly 3:40 a.m. UTC.

The reason for this according to Jameson was due to unusually fast block confirmation times.

Normally, miners on a proof-of-work blockchain like ethereum and test network Ropsten are required to manually upgrade their software in order to ensure the smooth continuation of a single chain.

According to Hudson, the majority of miners on the Ropsten blockchain did not upgrade to the latest software, since the time of the hard fork caught many developers off guard. This has resulted in a split of the test network between those mining on the upgraded chain and those mining on the outdated chain.

Last October, a similar event occurred after the activation of ethereum’s previous system-wide upgrade, Constantinople, which resulted in a temporary chain split on the Ropsten network lasting a few hours.

“The complicated part about proof-of-work test network is getting coordination between miners,” Jameson said in a call Monday afternoon. “Right now, we’re trying to run some miners to get Ropsten on the correct Istanbul chain.”

Jameson added that issues with the Ropsten network so far appear to be the result of poor miner communication, not flaws in the Istanbul upgrade code.

As such, how this temporary chain split will ultimately affect the activation of Istanbul on the ethereum main network is still to be determined. Ethereum core developers will have a call on Friday, Oct. 4 to discuss Istanbul’s testnet activation.

Hudson Jameson image via CoinDesk archives