Everyone knows that a pairs with t, and g pairs with c. on occasion, however, mis-pairings may occur. which mis-pairing would most closely resemble a normal pair (assume that all bases are in their most common unmodified form)?

It's more common to happen transitions( even though there are less possibilities) because of the shape of the molecules.
Transitions exchange bases of similar shape (swaps a purine for a purine or a pyrimidine for a pyrimidine), whereas transversions exchange bases of different shape( swaps a purine for a pyrimidine (or vice versa)).

A and  G are purines, C and T are pyrimidines.

possible transversions A-C, A-T, T-G, G-C
transitions are A-G or C-T

AG or CT pairs

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