Kim Wexler (Rhea Seehorn) writes a check to Howard Hamlin (Patrick Fabian) (for her law school student loans). The date on the check is March 4, 2003, putting this event six years before the introduction of Saul Goodman (Bob Odenkirk) in Breaking Bad (2008).
The Band-Aid box that Jimmy (Bob Odenkirk) takes from his father's old store appeared in the Gene flash-forward in Uno (2015) where it was stored in a box containing some of Jimmy's old mementos.
Jimmy's strumming the riff of Deep Purple's Smoke on the Water (the same song that played out of the Season 1 finale) on his new guitar, which is an autographed Ritchie Blackmore white Fender Stratocaster he got from the music shop.
Jimmy's fall/injury (i.e. the episode's title "Slip") in the music store is noticeably aided by CGI (which actually adds to the over-the-top humor of his landing).
When Chuck (Michael McKean) is in the supermarket, he says "Yellow bananas, green grapes, orange oranges". He is exercising an anxiety technique which involves focusing on your environment and naming things you see, this can help to ground a person and manage distressing thoughts in the moment.