We had a service that had a list API that was paginated. It returned a nextToken to specify the start of the next page of the results.
Internally we were doing a database migration to a completely different system and migrating one customer at a time. The problem was that if a customer was in the middle of a list call and had the next token with them which was generated from the previous database system, after migrating to the new database the older token would not be able to start from exactly where it should had the customer not been migrated. This was because it would not have all the information of the service to resume at the exact offset.
One option was to throw an error and let the customer retry the request; another option was to return some possibly duplicate items in the next page; none of these were good enough for both engineers and PMs and instead we decide to take up a bunch of additional work so that no customer would be impacted. This was 10+ weeks of additional work for the whole team but we did it because culturally it felt the right thing to do for the customer.
Note that the impact would have been tiny if at all. A customer would have to be in the middle of a paginated request and out migration system would have had to migrate that particular customer at that exact time and the impact would have been a few possibly duplicate items. But we didn’t know the actual impact of those temporary duplicated for a single call and we all agreed breaking changes like this are unexpected and cause customer to lose trust with us.