It is extremely unlikely that a fault proof will successfully challenge a transaction's results, because nodes that submit an incorrect result are heavily fined for it. But if it does happen, the transactions processed on OP Mainnet do not change. What changes is the merkle root for the state that is committed to L1. As long as the replica you use processed the transaction correctly, from your perspective the chain continues normally. The only case in which the challenge will affect is when:
- You used a replica that provided the inaccurate state
- The inaccuracy was specifically in part of the state that affects your transaction
Let's take an example. We have four people, Alice, Bill, Carol, and David transferring ETH between themselves.
| Action \ Balance post action -> | Alice | Bill | Carol | David |
| 1. Initial Balance | 100 | 100 | 100 | 0 |
| 2. Alice sends 50 to Carol | 50 | 100 | 150 | 0 |
| 3. Alice attempts to send 100 to Bill, the transaction fails because she doesn't have that much | 50 | 100 | 150 | 0 |
| 4. Alice sends 50 to David, which is successful | 0 | 100 | 150 | 50 |
Now, imagine that there is a fault and the sequencer thinks that after step 2 Alice still has 100 ETH. On the sequencer the transactions will look like this:
| Action \ Balance post action -> | Alice | Bill | Carol | David |
| 1. Initial Balance | 100 | 100 | 100 | 0 |
| 2'. Alice sends 50 to Carol, and for some reason the sequencer doesn't reduce her balance | 100 | 100 | 150 | 0 |
| 3'. Alice attempts to send 100 to Bill, the transaction appears to be successful | 0 | 200 | 150 | 0 |
| 4'. Alice sends 50 to David, which fails because Alice doesn't have it | 0 | 200 | 150 | 0 |
However, the 2', 3', and 4' balances only appear on the faulty sequencer and any replicas that copy their state from it (instead of calculating the state on their own). After the challenge is handled, all those replicas know not to trust that sequencer, and will take their state from a different node (or calculate it themselves), so they will have the legitimate balances, 2, 3, and 4:
| Action \ Balance post action -> | Alice | Bill | Carol | David |
| 1. Initial Balance | 100 | 100 | 100 | 0 |
| 2. Alice sends 50 to Carol | 50 | 100 | 150 | 0 |
| 3. Alice attempts to send 100 to Bill, the transaction fails because she doesn't have that much | 50 | 100 | 150 | 0 |
| 4. Alice sends 50 to David, which is successful | 0 | 100 | 150 | 50 |
Note that Carol is not involved in any of the faulty transactions, so her balance is the same in both cases. In real life almost everybody is like Carol.