I came across the mention of tuples, and had to look up what that was.
I came across a mention of tuples, and had to look up what that was.
I think that "a" is more logical here, because this was the first mention of tuples in the text, and therefore it was unknown both to me and the reader who is reading my text.
However, I found numerous instances of "came across the mention", like this one on Google Books:
A few years after writing Warrior Scarlet, I came across the mention in an old book of a flint celt that had been dug up on my stretch of the Downs.
Which option is better then? Is there no difference between "I came across the mention of /something/" and "I came across a mention of /something/"?