You opened this can of worms, now lie in it

I resent, and resemble, this

I laughed. Thank you! "You've opened this can of worms, now lie in it" is my fav.

You're shutting the bag after the cat's bolted

One I accidentally said a while back and now use for anything that's both unhinged and comes out of nowhere: Out of left pocket

You can beat a dead horse but you can't make it drink.

Oh yeah! How could I forget:
"Get two birds stoned at once"

Almost like carrying the horse with a cart.

That's a whole different kettle of wax

"This is a tough nut to swallow"

"It's not the end of the day"

I'm fond of "not the brightest cookie in the knife drawer"

Jemandem einen Bären über den Tisch ziehen.

Does a bear shit in the pope's hat?

get two birds stoned

We'll goose that cross when we come on it.

I love these. I have made some.

While you shouldn't sweat the small stuff, it's the details that make all the difference.

Variety may be the spice of life, too many cooks spoiled the broth.

Here's one I've heard. It was from someone's boss during a meeting:

C'mon, people, this is not rocket surgery!

From the Buffy-verse.

Harmony, the dumbest vampire.

"You're preaching to the horse's mouth."

It's all water under the dam.

What's sauce for the goose is good for General Motors.

"Yesterday we were at the brink of abyss, today we took a step forward"

we're peddling in the same shoe

My favorite one that everyone hates is "Even a blind elephant can remember to nut twice a day"

Blind squirrel finds a nut

Broken clock is right twice a day

Elephants never forget

Does the pope shit in the woods?

I use "not the sharpest brick in the tool shed" often.

You can lead a man to water but you can't teach him to be a horse

I always liked "Not the brightest bulb in the fridge" but I've also heard "coldest frog in the pot" means stubborn, but I'm not sure that's a malaphor.