This one still gets talked about, eight years later. Glenallen Hill, in the second inning on May 11, 2000, against the Brewers, slammed a Steve Woodard pitch onto the roof of the building at the corner of Waveland and Kenmore. Woodard was a pretty bad pitcher -- he gave up a Ruschesque 26 HR in 147.2 IP in 2000, and didn't even finish the year with the Brewers (he was traded to Cleveland in July).
Still, that's pretty damn impressive. That building has been there almost as long as the ballpark, and that is, to my knowledge, the only time anyone has hit a ball that made the roof. (The building has a much larger seating structure on top of it today than it did in 2000.)
2000 was a pretty bad year in Cub annals. They lost 97 games, third-most in club history. They were already 9 games under .500 and 7.5 games out of first place on May 11. And look at the starting lineup they trotted out that day:
But the Hill HR is memorable. Had Hill been given a regular DH spot by some AL team early in his career, he might have hit 300+ HR. It's surprising the Blue Jays didn't try this -- they really didn't have a regular DH when Hill first came to the majors. He's now a coach for the Rockies.