The possibility of a brokered convention.

Clinton didn't really end her campaign, she merely suspended it. By not releasing her delegates, they are still free to vote for her on the first ballot at the convention. So, if something caused enough superdelegates to switch to Clinton between now and late August, there is still at least the possibility of a brokered convention.

In defense of brokered conventions

"It's a long shot, of course, but political junkies can still hope."