You Have More Choices Than You Think
We can't move items backward in Kanban.
Of course you can! Whether or not you should will depend on what "additional work" means within your current process. In most cases, it either means:
Your process failed to complete or expose all the work needed to get a task to the Definition of Done for a given activity or for the process as a whole. Therefore, the work item should never have been pulled forward and needs to be sent backwards to be done properly within the appropriate activities.
You have exposed new work that is related to a given work item, but wasn't part of its original scope or Definition of Done. In such cases, you need to define the new work items, and handle those in accordance with your current working agreements for queuing and prioritization.
There are bound to be other edge cases as well, but these are the two most common. You're experiencing some dissonance about this because you seem to think work has to advance monotonically, when in reality your solution space can involve anything that:
- Doesn't invalidate your working agreements.
- Doesn't violate your WIP limits.
- Doesn't treat a one-off problem as a reason to chuck the whole process.
- Doesn't treat a recurring process problem as one-off problem to be glossed over.
If it's truly a one-off issue, stop the line until you can collectively resolve the problem. Otherwise, inspect-and-adapt your processes and working agreements to avoid similar classes of problems in the future.