Black Sea 704AD
Justinian II's ship is caught in a storm. In our history Justinian II regains his throne and soaks the Byzantine Empire in blood. He was also a second rate emperor as well. His subjects revolted again and killed him in 711. Meanwhile
Tiberius III droped the wasted effort to regain lost territories in the west and focused on defense against Eastern foes. Had his life been longer Byzantium would have been much stronger.
If Justinian II were to drown in the Black Sea and Tiberius III were to rule longer (and as Tiberius seems to have been far less brutal and vastly more tactful than Justinian) and less destructively, you might get a stronger Byzantine Empire for centuries longer.