Disadvantage on finding stuff with Perception is a big deal. When I ran Tales from the Yawning Portal, one of my players smugly said that he was taking Observant so he would find everything.
Then with his Darkvision, he would go ahead of the party to skulk about.
When he inevitably ran into a trap (we started with Sunless Citadel, and the first part is full of Kobolds), he seemed surprised. When it kept happening, he finally said "James, I have Passive Perception XX!", to which I replied "that's correct, you just missed the DC."
This went on for two sessions before another DM came up to me (this was in Adventure League) and asked if I was using the rules for passive perception correctly.
"Of course, he has passive perception X, and as you see here on page 5, the trap is DC Y."
"Uh, that's high enough for him to find it."
"Not with Disadvantage it isn't."
"One, Disadvantage doesn't affect passive checks, and why does he have it?"
I quickly pointed him at the rules for how advantage/disadvantage affects passive checks. "And he has disadvantage because he's using Darkvision."
"Huh, why? It lets him see in the dark!"
"It lets him treat darkness as dim light, see it says so right in his racial traits. Now if you go here where it discusses dim light, you'll notice he has disadvantage to his perception."
The DM blinked at the rules, as if he had never seen them in his entire life, in fact, he went to check his own rulebook. Then he wandered off. He came back a little while later and said "I checked online, you're right."
I wondered why he felt the need to check, but whatever. Then I watched him wander over to the Rogue player on the other end of the store (nonchalantly reading the store copy of the adventure...but that's another story).
A few moments later I heard a "WHAT?!" and I shook my head. The next session, he asked if someone had light or a torch. He then confronted me and asked why I didn't tell him he had disadvantage.
"It's in the rules, I assumed you knew."
"Nobody plays it that way!"
"I do."
Ever since then, any time someone says "Darkvision is OP!" I ask if they're using the penalties correctly. Because I have, and it's astonishing how many times players will blunder into an ambush thinking they're smart for not using light (while wondering why Darkvision equipped monsters in my games use light when not trying to be sneaky).
