expressions - One word to say to someone who's being in a state they're usually not (or doing something they never did)?

In my native language, there is a singe word to say to someone who apparently are doing something they never did before, or being in state they never were before. Like:

Mr. X had always been late. But today he came on time.

Mr. Y said, "[...], you're always late before."

Or:

Ms. A had never kept in touch with Mr. B. One day she dropped him a line all of a sudden.

Mr. B said, "[...]. What makes you call me? I thought you'd forgotten we're friends."

Or:

A student had always been so much lazy. But one day he finished all of his homework and listened to the teacher during the whole class.

So the teacher said, "[...]. You're being diligent. You usually are not."

I know that this condition is described by means as in you are being diligent, or you're not usually like this.. Or perhaps this is not you.?

But I need an expression for that (one-word expression, if possible), and I can only think of these:

• How odd!


• For the first time.


• What happened? (Or what's happening?)


• Hey, is this really you?

Well, I don't think I'm doing this right.

P.S.: What we say to this person (according to my mother tongue, Indonesian): Tumben!.

P.P.S.: I'm looking for an insinuating or joking expression, not swear-word interjections like the ones in the suggested possible duplicate.