Software development is prone to defects, even with strong done rules in place. Usually, dealing with operational issues (tickets) or ordinary bugs is a regular part of doing business, like email or other daily overhead. The effort required is already factored into the average work rates used to determine WIP limits and doesn’t need to be tracked on the signboard (assuming you already have online systems for tracking bugs and tickets). However, sometimes fixing a particular bug or an operational issue is as much work as other product improvements. These tricky issues are managed like any other work item. You create a note card, order it against the rest of the backlog, and slot it into its proper place. These complex issues typically need to be specified, implemented, and verified like any other product work.