Despite Edna being killed by Homer, she appears at Lisa's Ultranet Friend Requests. However, Lisa's behavior and the number of requests she has indicate that it's been a long time since she was last on the ultranet, so Edna may have sent the request before she died and Lisa hadn't seen it.
There are only 42 octuplets of the octuplets. If all of the octuplets had octuplets, there should be 64.
When Lisa enters the Ultranet she comments that it is Dr Seuss' birthday as indicated by the design of the Google page. Dr Seuss' birthday is March 2 and this episode takes place over Christmas.
Lisa and Homer refer to Prince Harry as a "ruthless tyrant" implying that he eventually ascended to the British throne, in which case they should have referred to him as King Harry, not Prince Harry.