"...are they bipolar?" or "perhaps they have MH issues?" as a potential explanation for odd or unusual behaviour?
I will declare my self-interest. My DH has severe non-psychotic bipolar disorder. He doesn't behave weirdly or inappropriately, he doesn't make people uncomfortable with his behaviour or draw attention to himself. Mostly, he struggles to get out of bed and go about a normal day so isn't crossing anyone's path other than mine and his medical/CMHT team's, has regular suicidal ideation (which you wouldn't know about unless you were as close to him as I am), suffers from night terrors (ditto) and keeps himself to himself.
It feels as though increasingly, if someone posts about or mentions someone's - friend or stranger - odd or bizarre behaviour, some well-meaning but way-off-the-mark soul will pipe up with a variation of the questions above. It frustrates me, it makes my DH's life harder if, on learning of his diagnosis, people assume he is going to behave strangely or inappropriately, and it makes me want to shout "you can be weird without being mentally ill, you know!"