Chain reorganizations

A reorganization happens when two miners find a block at the same height and the network briefly holds two versions of the chain's tip. Every node follows the branch with the most work, so when the other branch pulls ahead, nodes switch to it and the blocks they had become stale. Their transactions are not lost: most were also mined on the winning branch, and the rest return to the mempool to be mined again.

Most reorganizations are one block deep and settle when the next block arrives, and the deeper a block sits under later blocks, the less likely it is to change. That is why a payment is treated as more final with each confirmation, and why an explorer or index follows its node to the winning branch rather than keeping the stale one.