The ending of the episode with Grampa crashing the wedding is a spoof of the 1967 movie, The Graduate.
Grandpa performs the classical "roll dance" from Charlie Chaplin's The Gold Rush. He's even almost arrested for "copying" Charlie Chaplin.
The chalkboard gag is a reference to a copyright notice by Major League Baseball, warning everyone that it is copyright infringement to rebroadcast, retransmit or reproduce MLB programming without permission from MLB.
Despite the claims of the Blue-haired Lawyer, Grandpa's immitation of Charlie Chaplin isn't violating any copyright law, as The Gold Rush has been in public domain since 1953. The Blue-Haired Lawyer is very nitpicking with immitations and retellings.
The delivery guy with the animation cell has yellow skin when Bart looks through the mail slot but when Bart steps out he has darker skin. He keeps the darker color when he punches Bart but in the next shot he is completely yellow again.
Grampa shouldn't get gas bills, since he lives in a Retirement Home.
Grampa plays the organ at the wedding where Burns and Jackie are nearly married. How did he get up to that little room with the organ? There is no door leading into or out of the "organ room", and a sheet of glass in front of the "front" of it. Additionally, the "organ room" was not there in any previous episode. This was most likely intended as a one-time gag and not meant to be taken too seriously, though.