[rspeele]

You can roll back transactions for whatever duration of time you were able to outspend the network. Critically, that can include your own transactions.

You mine quietly, outpacing the regular network, creating your own longer chain. Perhaps you do this for an hour, several hours, or a day. The only transactions on this chain are those in which you send your own coins to yourself -- maybe plus some random transactions from the mempool you throw in to help cover your own tracks. Meanwhile you spend those same coins (UTXOs) on the "honest" chain.

Once you are satisfied, you publish your longer chain. By the rules of the network the longest chain is the true one. Honest miners immediately begin building their blocks on top of your dishonest version of events. Since you are now working together with them, blocks are produced quickly. The honest chain dies.

Chaos would ensue.

Anybody who received coins on the dead chain can try to republish the transaction to the mempool and hope it gets mined again, so they get their money again. But if the sender is fast, they might realize they have a double-spend opportunity here, and submit a higher-fee transaction in which they sent those coins to themselves. It's a race. Of course, for your own UTXOs that were already spent in the dishonest chain, the race has already been won. Those double spends are successful.