Bitcoin.com's mining pool, running Bitcoin Unlimited, recently accidentally mined a block greater than 1,000,000 bytes. This block is considered invalid under the current consensus rules and was subsequently rejected. This resulted in Bitcoin.com to lose the ~13.2 BTC block reward of that block and waste their miner's time and electricity.

This invalid block also resulted in many BU nodes being banned by other nodes for relaying an invalid block.

Note that this is different from normal block orphaning as this could have resulted in a consensus split, not just a normal small fork with all full nodes switching to follow the longest chain. The longest chain could have been built off of the invalid block due to BU and SPV miners making up nearly half of the network hashrate.

This block could have resulted in an actual hard fork and chain split due to BU miners and SPV miners. Some BU miners would have considered this block valid and thus mined on top of it. SPV miners would have received the block header and mined on top of that without checking the validity of the actual block itself until later. This could have resulted in a blockchain split similar to the July 2015 fork. Fortunately neither of those happened and there was no chain split.

The reason for this block is that BU changed some important code regulating the size of the block. Specifically, they removed something that reserved space for the coinbase transaction and thus once the transactions had been selected and the coinbase transaction were added, the size was too big. This bug appears to have been introduced here which was also committed directly to the source code with a Pull Request, so it is likely that the change was not reviewed.

See also: Reddit discussion