@Switch Blayde
That's when I learned there were major differences between AmE and BrE. I always knew about the z and s and o and ou and er and re, but I would change "tyre" to "tire" thinking it was a typo. Funny, no one ever complained.
For me, I was always aware of the typical word differences, and when you're unsure, there's always Google to help. But, I belatedly learned, already well into my writing career, that I'd somehow picked up an entire slew of British spellings for the longer, less regularly used words which I'm still struggling to overcome.
I assume I picked those up when reading many of the English (and Russian) classics in high school, and was never corrected in ANY high school or college class (probably since I NEVER studied literature).