A Bitcoin Unlimited node accidentally created an oversized block, which resulted in a temporal ban of many Unlimited nodes and financial losses for some miners.

According to a Reddit post, an anonymous bitcoin node created a block of 1,000,023 bytes. The current maximum blocksize limit in bitcoin system is 1MB, and a block exceeding this limit is considered invalid. Those miners who created this block will not be able to use their premium.

In the current case, the invalid block was a production of a node running the alternative Bitcoin Unlimited (BU) client. Resultantly, Bitcoin Core nodes banned a number of BU nodes that spread the confirmation of the invalid block.

It seems likely, that the invalid block resulted from a bug in the BU software.

“The impact of this bug is that in common mining configurations (including Bitcoin.com) this can result in somewhat oversized blocks, even when configured with a maximum of a million bytes,” wrote user nullc.

To avoid further complications, Reddit commenters suggest that BU programmers fix the bug asap and in the future use the peer-review system when making alterations to the code.

The invalid block was mined by bitcoin.com - the pool supported by Roger Ver and other notable BU proponents. The rejection of the block by Bitcoin Core nodes means that the pool lost over 13 BTC in premium.

Roger Ver reacted with an angry tweet:


Bitcoin Unlimited is the version of Bitcoin client that allows users to produce blocks of any preferred size. Theoretically, this may lead to a hard fork of the bitcoin system if a new block size is massively adopted by nodes. However, the number of BU supporters to the mainstream Bitcoin Core version remains relatively small. According to CoinDance, there are currently 457 BU nodes versus 4,796 Core nodes.

Meanwhile, a group of Core coders is developing the next stage of bitcoin scaling solution, including the long-awaited 2Mb block size limit. the project aims to increase the maximum block size limit to 2 Mb, while in fact helping to decrease the actual and average block size. This is necessary to reduce the Initial Block Download (IBD) cost and improve block latency.

Roman Korizky