How might umpires' use of instant replay have changed the course of baseball's storied history? Assuming the limited form we'll see initially, it's practically impossible to say. It's not that there haven't been plenty of important-but-questionable home run calls throughout the years. It's that most of them have been almost completely forgotten because before instant replay, nobody had any proof one way or the other.
There are photos of close plays in the infield, but before the 1970s, there simply wasn't any practical way to prove whether a home run was a home run or not. So what follows is a list of big plays, home runs and otherwise, that might well have been reversed had instant replay been used liberally during the past century or so.