The original title for the show, We Are Mongrels was changed because of worries people would confuse the show the similary titled We Are Klang (which was airing at the same time) and also because none of the characters were actually mongrels.
Rejected titles included I, Nelson; Humans! Everywhere!; Never Been Stroked; The Garden Gang and Undergrowth.
When Russell Howard first heard that his name and lazy eye had been used as a joke in the first episode of Mongrels, his reaction was: "S***ting hell, my genetic defect has become a punchline". However, he mistook the character of Nelson for another cat.
Source: Mock the Week, Episode 9.6.
In Episode 1.3 ("Marion the Young Lover") one cutaway shows the pub under a different name. While the pub is usually called The Lord Nelson, in one scene it is shown as "The Royal Standard".
During the song Middle Class is Magical, the band Vermin is spelt differently. It is spelt as "Virmin".
Source: Mongrels Series 1, Episode 7 DVD Commentary