When I am talking about fibrous material, like individual strands of carbon in a composite, I naturally type “fibre” but when I talk about nutrition or the internet it’s “fiber”

I also tend to spell armor armour and color colour despite being American.

Oh and I write grey instead of gray.

I also catch myself writing units like metre and litre instead of meter and liter sometimes.

It really all depends on if there’s a spellchecker turned on that will tell me I’m spelling things wrong.