Parents feeding their baby cola in bottles and smoking while pregnant are two things that usually cause me to make assumptions

Associating with arbitrary groups, such as football fans, nationalists, wearing certain clothing brands

Not being able to entertain ideas. "What would the world be like with 100% renewable energy?" "Would basic healthcare for every person help our country?"

I tried to explain the 4 day work week to someone that gets paid by the hour. You make the same money but work 4 days a week instead of 5. Insisted he got paid less. Had to explain like a Bingo card with a Free Space, 1 day he is paid even if he stays home.

I see this in a lot of places I do work:

Toolboxes covered in union stickers, AND Trump stickers...

Ironically, defending arguments using scientific studies and experiments, but not being able to think critically about the methodology used or what the results mean. Too often people will cite scientific literature based off the title and MAYBE skim it. Trying to have a discussion with them will usually result in them calling you anti-science.

A good example is the pseudo-scientific belief common within incel circles that women can store and absorb dna from past sexual partners and that their children can then have more than one genetic father (an excuse to shame sexually active women while fear mongering about cuckoldry). If you track down the source the study actually explicitly explains exactly why this isn't the case.

Cigarettes

Confuses to, two and too.

Also their, there and they're

Seeing a pirate use unsafe sites

Not listening to other people's opinions and ideas

Believing the earth is flat

Black/white thinking. Everything is either bad or good, the problem or the solution.

Referencing the Dunning-Kruger effect in casual contexts. Most people who refer to it, have not really read about it enough to be qualified to use it.

taking Ayn Rand's work seriously. five seconds of critical thought and her entire philosophy comes crashing down

Using terms like 'u', 'ur', etc when writing. No one charges by the letter, it's simply lazy.

Went on a trip a while back with a relative, here are some of the things that they belived/behaved like(sorry for the long text):Needless gossiping and thinking that other people's achivements threaten your credibility. Anger/apathy/being dismissive as default, snarky useless remarks, belittling other people any chance possible, trying to seem superior through "presenting intelectual", playing "the devil's advocate" just to look edgy and be unnerving, beliving propaganda over science, not apologising when needed, lacking self awareness and being selfish constantly, not going to therapy&health check ups, making poor spirited jokes about other people, not respecting people's space, touching them without permission,being disrespectful on purpose to other people(and to yourself), and the phobias(homophobic, biphobic, transphobic, xenophobic),misoginistic,misandrist, ableist, rascist,antisemitic,heavily nationalist attitude, thinking the world revolves only around you and that other people are just tools&objects, not cleaning after yourself, expecting other people to take care of things for/accompaning you as if they have an obligation, thinking violence settles things, refusing to pay people because "it's for exposure" and "they should do it for the experience/self fuffilment", loud music in public spaces(please use headphones), dismissing people's religious practice just because it isn't exactly how you would do it, pressing other people to live the life you want to but not living the life you want to live yourself, being needlessy pessimistic and cynical about the future(guilty), not being bewildered about earth and not wanting to learn or practice random things(who has time to be hateful when you see a beautiful leaf that has just sprouted?), lack of wanderlust.