I was going to answer this but I can't remember.

Everyone's names.

The phrase "Baader-Meinhof effect"

I love pointing out examples of it but always forget the name, leading to an awkward moment when I try to explain it

I have ADHD, so pretty much every word when I need it most.

this implies one. all sorts of words the moment I need them.

I have no idea why, but convention. And not a thing where nerds like me gather to dork out about something, but a scientific standard. Whenever I'm explaining something, and someone asks why it operates that way, I'm always like, "it's that way by.... uh... y'know, it's always been that way." No clue why I always blank on that word specifically.

deflagration

I had to look it up for this post. My brain's inability to recall the word for detonation velocities lower than the speed of sound wasn't an issue until rotating detonation engines started to make news and I've needed to explain the difference between explosions and deflagrations to people.

Funny you should ask. And that's the thing, I can't remember.

I forgot. 🤷🏻‍♂️

Defenestration

Gets me every time I need it...

Ostention, which I occasionally use in its folkloric sense, is one that I can hardly ever bring to mind at the critical moment.

ma ja pa hit

oh, um... shit, it's right on the tip of my tongue...

Empathy and sympathy. I know they are different, and I know how they are different, but I always forget which is which.

Effect/affect

I'm better at writing than talking because it takes me so long to think of so many words.

today I paused for 30 seconds trying to remember "second line of defense,"

and paused again for a really long time trying to remember what this thing is called I plug my phone into recharge when I'm traveling, oh yeah it's called a "power bank."

The one that means:

When you find unexpected good fortune.

I always remember the word "providence" when I mean the other one, without the religious connotation. Like kismet.

It's Serendipity btw.

I am very thesaurus-minded and express myself precisely when possible, so I don't have any words I just forget, but once in a while I might not be able to find a word that embodies what I'm trying to get across, which is always frustrating as someone whose first language does have an equivalent to the missing word in question.

"Idiomatic" and "Wirth's Law"/"May's Law" come to mind.

Of course now I have no problem bringing them to mind.

I forgot

callipygian

Ladle. Big soup scoopy thing. Never remember this word. Terribly specific.