Animals, babies and mental patients could all see Al.
Electricity could futz with the link between him and Sam, screwing up Sam's separation from his host body's personality and occassionally switching Al into a host and Sam back to the Quantum Leap facility.
And there was an Evil Leaper called Alia, whose holographic guide person was called Zoey. They were from further in the future than the Quantum Leap project and could control leaps into specific people, as opposed to Sam's random jumps. They went through time wronging what once went right.
In the finale, Sam leaps into this bar on the day of his birth, but it's all screwy. He sees people leaping in and out, people being remembered and forgotten and they've all got names of people he's known. The bartender, Al, tries to explain things to him but it's pretty confusing and all open to interpretation. But eventually Sam seems to understand and leaps out, with the series closing: "Dr Beckett never returned home".
In an alternate ending, he makes one last leap to Al's first wife's house in the 70s while Al's still a POW in Vietnam - but he leaps as himself, not into someone else's body - and he tells her Al's still alive and to wait for him. This fixes Als entire life, erases about 7 failed marriages and he ends up happily married with about 9 kids. It also presumably erases the Quantum Leap project from history and leaves Sam leaping forever through time, putting right what once went wrong.
Very cool.