Harry Harrison was a guest at the first con I ever attended outside Texas, Philcon in 2003. That whole con was quite an experience for me - my learning curve then was very steep. I enjoyed his panels and learned a lot - although, after having lived in Texas 18 years, I found his clipped speech and fast pace, especially when he got excited, to be daunting to follow.
The following summer the wife and I drove up to Lawrence, Kansas, to attend the Campbell Conference. That year, 2004, was the last where the members of the Science Fiction Hall of Fame were inducted at the same time (the event has since moved to the sf museum in Seattle).
The living honorees were Brian Aldiss and Harry, We arrived in Lawrence just in time for the dinner, and as I rushed into the student center - worried that we were running late - I saw a pair of old timers in tuxes heading for the door from the opposite direction.
As I ran up, I realied they were Aldiss and Harrison, which floored me. In a clumsy attempt to be a gentleman, I grabbed the door to hold it open for Aldiss, who was first. But as I walked around him, I stepped on the back of his shoe and gave him a "flat tire". (My wife tried to make me feel better later by pointing out that Aldiss was wearing house shoes).
I don't think Aldiss was terribly happy, and I was so embarrassed I ducked both of them for the rest of the weekend.
I've been a panelist at at least two conventions over the past few years where Harrison he has also been a guest, and I've steered clear. I hope he doesn't remember me!