Mike Hearn and Gavin Andresen release Bitcoin XT, the first serious attempt to fork Bitcoin by increasing the block size from 1 MB to 8 MB. The release ignited the "block size war" that would consume the Bitcoin community for the next two years. Supporters argued larger blocks were needed to scale Bitcoin as a payment system, while opponents warned it would centralize the network by making nodes more expensive to run. Bitcoin XT ultimately failed to gain sufficient miner support. The debate laid the groundwork for SegWit, the Lightning Network, and the eventual Bitcoin Cash fork in 2017.
Wikipedia: Bitcoin XT