Is it ever correct to say "something in a list"?
Searching for "in a list", quoted, returns "About 800,000,000 google results." So it would be mind boggling if that were not considered "ever correct."
Searching for "in the list", quoted, returns "About 357,000,000 google results."
The very first result from the search is Mirriam-Webster answering exactly this question, "Is it correct to say "on the list" or "in the list"?"
...it is customary and correct to say on the list. Most often, "in the list" is used after some form of the word include, as in the examples shown below.
There are other cases. For example, in Computer Science a variable could be "in an array", or similarly "in a list".
Let's analyze this step by step in more careful detail. Consider the sentence "Put his name on the list." Then, just as an experiment, change it around to "Put his name in the list." The problem with "in" here, is that it implies "inside", such that on a list of 100 people, the new name would go at position number 25, which is "in" the list. However that was not the intention. Almost certainly when you add a name to a list, it would be appended to the end of the list. Being at the very end, it's not quite fitting that it's "in" or "inside" the list. Rather, it's at the end. In fact, the name will be "on the top" or "on the bottom" of the list after it's added. Therefore, "on" fits the situation better.
In the future this same name will literally be "in" a long list of names, somewhere in the middle. If the words "in" or "on" were used in their most basic sense such as "on the table" or "in the pile", then the word "in" would actually be more appropriate when finding a name in a long list of names. So, why does "on" continue to be the right choice? Because it has shifted into an idiomatic usage where discussions of lists are concerned. You just keep using "on" in the context of lists, even where "in" might make more sense.
Yet, from time to time, "in" is still going to appear in sentences because the literal meaning of "in" applies to something within a collection of objects. In conclusion, it appears that "on" is used in an idiomatic sense, while "in" is used in a more literal sense.