The song "Let's Run Away Like Bonnie and Clyde" that played during the end credits of Timeless plays in the opening credits of Fetzenleben, and its melody is used as a re-occuring musical motif in the film.
In the sitcom-scene, the angry husband is wearing the very same squirrel-costume worn in the finale of "Killer-Squirrels". Additionally, Konstantin has a Killer-Squirrels poster hanging on the wall.
As Arnold and Jane ride in the train, one of the passengers is briefly seen watching "Mutant Calculator" on his smartphone. Additionally, when Konstantin enters his apartment, we briefly see the calculator-costume from Mutant Calculator standing near the entrance, as well as a poster for the film.
In the opening scene, the table and the set are the same as in "Hollow Date". Additionally, a poster of "Hollow Date" can be seen in Konstantin's apartment.