The team I'm in had our 5 next sprints all planned out, which is something the Greenhopper planning UI somewhat encourages by saying "Plan some sprints".

Somebody accidentally deleted the second one by hitting the subtle grey in the top corner of the sprint.

Re-adding it again is impossible. We have to rename all the sprints and shuffle the work down each one. Due to a single mis-click.

If adding sprints to the middle is impossible, I suggest that deleting them from the middle also should be.