I remember how, at one point during my mission in Greece, I was in a travel agent's for some reason and heard a (female) employee there say "Δεν είμαι σίγουρη" (I'm not sure).
My initial reaction was to be a bit startled since I'd have expected "Δεν είμαι σίγουρος"—the same sentence, but with the dictionary form of the adjective, which has the male ending. My next reaction was, "Oh, that makes sense—that Greek adjectives would inflect for gender also in predicative position".
I suppose my first reaction was conditioned by two things: having learned "Δεν είμαι σίγουρος" essentially as a set phrase (which worked for me, since the male ending of the dictionary form of the adjective was appropriate for me), and, probably more importantly, coming from German, which inflects adjectives in attributive but not in predicative function.
That is, while adjectives show the gender if they precede and modify a noun (ein großer Mann, eine große Frau, ein großes Kind: a big man/woman/child), they do not inflect if they are used predicatively (der Mann/die Frau/das Kind ist groß: the man/woman/child is big); there, only the stem of the adjective is used. So, in German, both males and females would say "Ich bin nicht sicher". However, this is not the case in Greek; adjectives inflect for gender no matter which role they fulfil, so you'd have "ένας μεγάλος άνδρας/μια μεγάλη γυναίκα/ένα μεγάλο παιδί" and "ο άνδρας είναι μεγάλος/η γυναίκα είναι μεγάλη/το παιδί είναι μεγάλο".
It was still a bit of an epiphany for me.