The contents of the written invitation from The League of Superheroes can be seen as Arthur reads them aloud, and they are completely different from Arthur's spoken words. Additionally, the letter refers to the League of Superheroes as "The Legion".
When The Immortal phones the Pope - presumably Karol Wojtyla in 2001 - he attempts to speak to him in (terribly mispronounced) Russian, rather than in Polish. (It could, however, be interpreted in-universe as yet another example of The Immortal's stupidity).
The "metal" and "armoured" costumes of the superheroes can often be seen bending and creasing throughout the series; it is particularly often visible when The Tick sits down in Arthur's armchair inside the "headquarters".
The Tick's costume features antennae. Ticks are arachnids, not insects, and arachnids have no antennae. This is likely another gag about how clueless and incompetent Tick is.