Seven
Seven
[Megan and Cueball are talking.] Megan: Can you name all the dwarfs from Snow White? Cueball: Sure, there's, um... Cueball's thoughts: Sneezy, phylum, Europe, sloth, guacamole, data link, Colossus of Rhodes I have this problem where all sets of seven things are indistinguishable to me.

In this comic, Cueball (or perhaps Randall ) says he can't distinguish between sets that have exactly seven objects. This leads him to exchange the items in the sets without noticing, to the point where, when attempting to list a single set, each item mentioned actually belongs to a different set.
This is shown in the comic when Megan asks Cueball to name the seven dwarfs from Snow White and the Seven Dwarfs , a task some people might find difficult, although they would not just choose words from other sets of seven to fill in the gaps.
The title text reveals that even a trivial set of seven items, like the days of the week, also goes completely wrong.
The comic may be related to the oldest set-theoretic definition of the natural numbers , in which for each natural number, an equivalence class is defined over all sets which contain the same number of items. As Cueball is known for mathematical thinking , he could be presumed to have taken the underlying equivalence relation to heart, and (over)applying it to real life, genuinely judging sets to be identical if they all contain N objects.
The number seven being the number for when sets become indistinguishable is possibly a reference to Miller's law .  Especially considering that this is a law dealing with human memory, which Cueball is having issues with.  However, this law refers to elements within the same set becoming indistinguishable, rather than the indistinguishability of different sets of the same size - indeed, its original tests involved either distinguishing between the items, or repeating them back in the correct order .  But then again, that might be part of the humor.
The number seven has culturally been regarded as a special, magical or holy number , which contributes to the large number of familiar sets of seven that make this comic possible. This proliferation of well-known sets of 7 items could be another reason why Randall chose to use the number seven in the comic.
In 1554: Spice Girls the game continues with Cueball saying that it is now Megan's turn and then he asks her a similar question regarding the names of the Spice Girls. Her problem is then that she simply finds different sets of five and then just adds Spice behind each of the words of that set.
For each of the seven lists below, the relevant item's traditional position within its own list of seven, according to Wikipedia, is not necessarily equal to its position on the list in the comic. For some lists the position is equal, but not for all. For instance Sneezy is traditionally never mentioned first amongst the dwarfs since the leader Doc normally comes first. But "phylum" is the second major taxonomic rank as is "phylum" the second item on the list in the comic.
The seven "dwarfs" mentioned and their relevant sets of seven are (items in the set are written in bold):
The title text extends this saying he also does the same with the set of the seven days of the week. There are several sets of 7 featuring the name Electra , which are all listed in the table below.
The sets Cueball's "days of the week" come from are:
[Megan and Cueball are talking.] Megan: Can you name all the dwarfs from Snow White? Cueball: Sure, there's, um... Cueball's thoughts: Sneezy, phylum, Europe, sloth, guacamole, data link, Colossus of Rhodes I have this problem where all sets of seven things are indistinguishable to me.
