Sarcastic way to say something is unexpected

One could say "Now I've seen everything."

According to the OED:

used hyperbolically to express surprise at something novel, outlandish, etc.

I would say this is a bit milder than sarcasm, but could be taken in a joking manner.

Alternatively, expressions such as "Who are you and what have you done with my husband?" may apply in this instance.

A common American quip to and out-of-character action is

Who are you and what did you do with my husband?

Typical example -

Who are you, and what did you do with my husband?

Sometimes my husband does things that surprise the heck out of me. Like today. I came home from my internship just as he was pulling in to the driveway. He's never home that early, so I was curious about why he was there.

Me: Hey, Honey! What are you doing home already?

DH: I came home to do some chores before I go do my recon tonight.

Me: Chores??????

DH: Yeah. I figured I'd take my clothes (dirty laundry) down to the basement.

Me: (Externally I said...) Oh. Okay. Should I mark it on the calendar? (Inwardly I said...) It's about damn time!!!

DH: Ha. Very funny.

What y'all don't know is that my husband's side of the bedroom is ALWAYS covered in his clothes - clean and dirty. I do the laundry and just add to his pile of "who knows what" on top of his dresser. Every now and then I say, "Honey, if you want your clothes washed you need to take them downstairs." It has taken me YEARS to get to the point where I don't nag him on a daily basis about his "bedroom mess."

BTW, not only did DH take his clothes down to the basement, he CLEANED half the basement! Now I have a clean, somewhat organized laundry area. Even though the 3-section clothes sorter no longer has "sorted" clothes and it is 6 feet from the washer/dryer, it's clean. And I didn't have to do it myself or even ask DH to do it! Time to celebrate!!!

The "turning point" from my nagging occurred when I returned from SBLive at Hood. When I got home DH had taken the kids to dinner so I could have some "down time" after my trip (this, in and of itself, was a surprise). And when I walked in the door, the house was IMMACULATE! (surprise # 2). So I walked to the bedroom to fulfill my OCD need to unpack right away (that drives DH nuts!) and to my surprise (# 3) the bedroom was CLEAN!!! No clothes on the floor. None on the dresser. All the various odds and ends had made there way "somewhere," but there wasn't anything out on the dresser, headboard, or floor. I seriously almost fainted.

I took a few moments to bask in the glory of a clean bedroom. I had no idea how long it would last or if I would ever see it again.

When DH came home I gave him a huge hug and kiss and thanked him for all of my surprises. Later I asked him why he never does this stuff when I'm home...

https://www.military.com/spousebuzz/blog/2007/09/who-are-you-and.html

Her husband's unexpected kindness made her do a double take.

"a double take" - "a delayed reaction to a surprising or significant situation after an initial failure to notice anything unusual —usually used in the phrase do a double take." from MW

First Known Use of double take: 1930, in the meaning defined above.

You do a double take to make sure something is not an illusion. Below is good example of "a double take", from "Double Take", a book of short stories by Roger Russell.

Emanuel did a double take. Taking a big breath he looked to another young man sitting in the group. “I just saw you before the meeting and you were working on the shuttle.” Alexander shook his head. “No, that was my older twin brother, ...