In the example traps pp. 240-241, I noticed that the two pit traps (camouflaged and spiked) have no save or other defense (like the “AC Save” of the arrow trap). I noticed it, but thought maybe that’s just the way it was, but then I re-read the description of the Acrobatics proficiency, and it says that it grants a +2 to saves vs pit traps (among other things).
So now, with the two anomolies combined, I’m wondering if pit traps should grant a saving throw to avoid them (Blast & Breath, probably?)
They only go off on a 1 or 2 specifically when a player does something that might trigger it. This is the representation of players using “exploration movement” and moving a max of 120’ in 10 minutes because they are watching their footing, being careful, etc. Now, if someone is in combat and steps over a pit trap, you might rule it automatically triggers. But out of combat, the 3+ could represent either the character noticing some type of tell and “not chancing it” or possibly a hinged pit getting wedged and not properly deploying.
I apologize. This is a case where the rules seem like they are more rigorously defined than they actually are.
The official rule for Acrobatics proficiency is "The character gains +2 to saving throws where agility would help avoid the situation." The phrase "such as tilting floors and pit traps" is intended to help the judge visualize what sort of situations those might be. But if the pit trap doesn't afford a saving throw in the first place, a +2 bonus won't matter.
The pit traps detailed on p.240-241 do not afford a saving throw, but have only a 2-in-6 chance of going off. If you invent a different type of pit trap that does afford a saving throw, Acrobatics would apply.
Acrobatics would likely also apply to:
Save v. Blast to avoid bricks from the ceiling
Save v. Blast to avoid portcullis falling suddenly downward