There are a number of occasions that spring to mind,from seeing Comet Halley and SN1987A in my 60mm refractor, seeing NGC 253 in a 19.5"scope
The transit of Venus Twice and of course the spy satellite separation at camp Duckadang a few years ago as mentioned by Graham in post #8
But for shear excitement I cannot go past Comet D/1993 F2) Shoemaker Levy-9 on July 16/17 1994.
I heard on the ABC radio that Greg Bock had seen a piece taken out of Jupiter in his 72" scope
(obviously focal length)
I rushed outside and set up my 20cm Celestron SCT, and gave out the most excited yell I have ever made in all my time in Astronomy.
I watched over the nearly one month both Day and night the impacts and their developments and demise.
Such an exciting thing to realize that one is watching something that had only happened 33 minutes before I was seeing it.
There are others that deserve a mention,but I don't want to bore you.