The Leap Back - June 15, 1945 - S4-E1
Corrected entry: It is accepted canon that Al sees Sam as the person he has leapt into, and Sam of course sees Al as Al. Therefore, at the beginning of this episode, Al would have been shocked to see Sam standing next to him, while Sam would not have recognized Al, but rather would be looking at Tom the soldier that Al has leapt into. However, they are unaware of the role reversal until Al walks into the cannon on the lawn.
Correction: While it was canon in early episodes (notably "What Price Gloria") that the Observer saw Sam as the Leapee, by the time of "Miss Deep South" (which originally aired well before "The Leap Back"), it is clear that a change has been made so that Al sees Sam as Sam. In that episode Al says that Sam looks "like Scarlett O'Hara on steroids." So for Sam and Al to initially not realize that something strange is going on at the beginning of "The Leap Back" is not a plot hole.