On August 26, 2016 someone noticed that their Classic node had been forked off of the “Big Blocks Testnet” that Bitcoin Classic and Bitcoin Unlimited were running. Neither implementation was testing their consensus code on any other testnets; this was effectively the only testnet being used to test either codebase. The issue was due to a block on the testnet that was mined on July 30, almost a full month prior to anyone noticing the fork at all, which was in violation of the BIP109 specification that Classic miners were purportedly adhering to at the time. Gregory Maxwell observed:

That was a month ago, but it’s only being noticed now. I guess this is demonstrating that you are releasing Bitcoin Classic without much testing and that almost no one else is either? :-/ The transaction in question doesn’t look at all unusual, other than being large. It was, incidentally, mined by pool.bitcoin.com, which was signaling support for BIP109 in the same block it mined that BIP 109 violating transaction.

Later that day, Maxwell asked Roger Ver to clarify whether he was actually running Bitcoin Classic on the bitcoin.com mining pool, who dodged the question and responded with a vacuous reply that attempted to inexplicably change the subject to “censorship” instead.

Andrew Stone (the lead developer of Bitcoin Unlimited) voiced confusion about BIP109 and how Bitcoin Unlimited violated the specification for it (while falsely signaling support for it). He later argued that Bitcoin Unlimited didn’t need to bother adhering to specifications that it signaled support for, and that doing so would violate the philosophy of the implementation. Peter Rizun shared this view. Neither developer was able to answer Maxwell’s direct question about the violation of BIP109 §4/5, which had resulted in the consensus divergence (fork).

Despite Maxwell having provided a direct link to the transaction violating BIP109 that caused the chain split, and explaining in detail what the results of this were, later Andrew Stone said:

I haven’t even bothered to find out the exact cause. We have had BUIP016 passed to adhere to strict BIP109 compatibility (at least in what we generate) by merging Classic code, but BIP109 is DOA — so no-one bothered to do it. I think that the only value to be had from this episode is to realise that consensus rules should be kept to an absolute, money-function-protecting minimum. If this was on mainnet, I’ll be the Classic users would be unhappy to be forked onto a minority branch because of some arbitrary limit that is yet another thing would have needed to be fought over as machine performance improves but the limit stays the same.

Incredibly, when a confused user expressed disbelief regarding the fork, Andrew Stone responded:

Really? There was no classic fork? As i said i didnt bother to investigate. Can you give me a link to more info? Its important to combat this fud.

Of course, the proof of the fork (and the BIP109-violating block/transaction) had already been provided to Stone by Maxwell. Andrew Stone was willing to believe that the entire fork was imaginary, in the face of verifiable proof of the incident. He admits that he didn’t investigate the subject at all, even though that was the only testnet that Unlimited could have possibly been performing any meaningful tests on at the time, and even though this fork forced Classic to abandon BIP109 entirely, leaving it vulnerable to the types of attacks that Gavin Andresen described in his Guided Tour of the 2mb Fork:

“Accurate sigop/sighash accounting and limits” is important, because without it, increasing the block size limit might be dangerous… It is set to 1.3 gigabytes, which is big enough so none of the blocks currently in the block chain would hit it, but small enough to make it impossible to create poison blocks that take minutes to validate.

As a result of this fork (which Stone was clueless enough to doubt had even happened), Bitcoin Classic and Bitcoin Unlimited were both left vulnerable to such attacks. Fascinatingly, this fact did not seem to bother the developers of Bitcoin Unlimited at all.