Anything missable about getting 100%

[Ash//Fox]

Title.

[DefraggedSSD]

Nope.

[Sgt. Pepper]

UFO's

[Vazkulator]

what is this?

[globefish23]

Nothing to miss to get the 100% achievement and T-shirt.
You only need a certain amount of random events, stunt jumps and bridge/knife flights.

Although, if you are a completionist like me, and want to get all items on the checklist, there are 2 random events that can indeed be missed.
They both don't have the usual blue blip on the mini map and have some limitations when they can be done.

--Simeon Yetarian:
Can only be done between the missions "Friend Request" and "The Jewel Store Job", when you come by his car shop as Franklin or Michael.

--Altruist Cult Shootout:
Can only be done by delivering 4 people from certain random events to their camp for cannibalistic feeding.
If you completed those events already, and don't have enough people, you can complete this random event and the associated achievement.
Pairs of people count as 2 people.

Note:
Several of the random events people give you valuable bonuses (heist crew members, golf partner, booty call, big money reward), so feeding them to the Altruists will deny you these bonuses.
There are still enough people with no real bonus to get the Altruist event/achievement.
https://www.gamefaqs.com/boards/634490-grand-theft-auto-v/67439297