In the store there is a display of "Famous People from Modesto Jar Candles" decorated with the faces of George Lucas, James Marsters and Timothy Olyphant.
When Marina says "She knew the consequences, and she still licked her paintbrushes" she is referencing the Radium Girls. The Radium Girls worked at a factory painting clock numbers and dials. They were encouraged to lick the paintbrushes for finer lines, thus injesting radioactive paint. They died horribly of radiation poisoning.
When Penny 23 prevents Stoppard from attempting to banish him & Marina, that's not a magical plant he's using but just an ordinary dandelion from another time-line, and its many seeds would would cause the same interference that they both did.
Camryn Manheim (Shelia) participated in the Deaf West production of the musical, Spring Awakening as three separate characters (Frau Bergmann, Fräulein Großebüstenhalter, and Fräulein Knuppeldick) in 2015, speaking the signed parts by Marlee Matlin and signing her own parts as well. Marlee Matlin played the character of Harriet on this show.
When Marina and Penny are in the alternate reality Brakebills, specifically the Physical Students cottage, which is completely abandoned with signs of arson, Marina says that "I cannot believe a bunch of militant muggles beat me to burning down Brakebills." 'Muggles' is the term for non-magic humans in the Harry Potter franchise.