A mistake I used to make all the time was mixing up 'he' and 'she' when I spoke English. I'm from Shanghai and I work in logistics, and in spoken Mandarin 'he' and 'she' sound exactly the same, so my brain simply didn't separate them.
I didn't really notice it until about three years ago, when I started having video meetings with a team in Manchester. One day I was describing my manager, a woman called Mei, and I kept saying 'he'. My British colleague Tom eventually asked, quite politely, whether Mei was a man or a woman, and everyone laughed. I was quite embarrassed, because I realised I'd probably been doing it for years.
To fix it, I tried a few things. I recorded myself describing people in photos, like family members and celebrities, then listened back and counted the mistakes. I also asked Tom to give me a small signal whenever I got it wrong in meetings. And I started slowing down slightly before pronouns, which felt strange at first but helped a lot.
The reason it was so difficult to stop is that it's not really a knowledge problem. Obviously I knew the rule; any beginner knows it. But when you're speaking fast and thinking about the content, your brain falls back on the patterns of your first language. In written English I almost never made this mistake, because I had time to check. Speaking happens too quickly for that. It took months of deliberate practice before the right word came out automatically, and even now, when I'm tired, it occasionally slips out.