We are using one project with an scrum board and five lanes (work, parking, processing, quality control, done), having an maximum of two allowed tickets in processing lane.
We have quite some stories with subtickets.
When using this board in a very small team of just two developers it happens regularly that subtickets moved to parking or processing get invisible. You can go to the story and click on the subticket link to open the details page for the subticket, but it is not shown in the board and the max ticket per lane counter does not count the invisible ticket.