If you take an issue that is in the current sprint backlog of project A and move it to project B it creates a variety of problems. When you look at the scrum board for project B you will see the sprint name from project A listed in the sprint filter list.

We are seeing a variety of serious issues with sprints that might be caused by this but we can't be sure. We have had sprints in different project using the same sprint ID. We then had sprints in a project get closed or deleted when sprints with the shared ID in other projects are either closed or deleted. It is creating a great deal of havoc. The only thing we have repro'd reliably is appears of sprint filters across scrum boards.

Repro:
Create projects A and B, each with their own scrum boards and backlogs
Create Project A Sprint 1 and Project B Sprint 1 in the respective boards.
Add stories to the sprint backlog for project A and start the sprint
Move a story that is in the current sprint of project A to project B
View the board for project B

Results:
Project A Sprint 1 should be listed on the board for Project B.

Workaround

Remove the issue from the Sprint it is in before moving it to the new project. This can also be done after it is moved by looking at Plan Mode, right-clicking the issue, and choosing "Remove from Sprint".