This episode marks the first time we see what a Grimm's eyes look like to a wesen when they are woged
The first 'ratboy' is named Rizzo. In 1969 the movie 'Midnight Cowboy' gave us Ratso Rizzo; the 1980 season of 'The Muppet Show' gave us Rizzo the Rat, named after Ratso Rizzo. The tradition continues.
This episode opens with the quote, "Rats! They fought the dogs, and killed the cats." This is from the Pied Piper of Hamelin by Robert Browning.
The opening quote is from the poem "The Pied Piper of Hamelin" by Robert Browning. The season one episode "Danse Macabre" also had an opening quote from this poem as well as various plot elements. That was the fifth episode of season one (and the fifth episode total); this is the fifth episode of the fifth season. Also, this episode is basically cats (klaustreiche) versus rats (reinigen), while the main character of that one was a reinigen who wore a large cat head as a disguise.
Continuing the running gag of including season and episode numbers in the show, when Nick looks at Truble's fake driver license with the name "Lauren Cole" the address lists her apartment number as "Apt # 505" and this is season 5, episode 05.