My dad has strange pronunciations for a lot of words, but maybe my parents always pronounced "then" and "than" so clearly that I never mistook them for the same word.
The first time I ever saw anyone get it wrong, that I recall, was in my sophomore year of high school. That was back in 1985/1986. My best friend and I both got modems for our computers and started to log into SLC area message boards. I noticed that he routinely made the same typo when writing the word "than" and asked him about it. The A and E are on the same end of the keyboard, but they're far enough apart that hitting one instead of the other accidentally shouldn't be so common.

He thought I was making fun of him for being stupid and freaked out on me. It was then that I recognized that people actually sometimes pronounce them the same way and that his error wasn't a typo but rather an issue of vocabulary.

Yeah, back then we didn't see each other write so much so it was easier for something like that to fly under the radar.
There as no texting or WWW. Even nerds on a BBS was a new thing.