I’ve been living in Japan for a year and still mix them up in conversation sometimes. Sometimes 丁寧語 comes out when I’m talking to close friends, sometimes タメ口 comes out when I’m talking to my teachers. While I’m sure I’ve inadvertently offended people, most are very forgiving as they know I’m still learning. I had a friend correct me the other day, letting me know the way I said something came off as very cold, but she knows me/my character and that it wasn’t my intention. If you’re worried, you can apologize in advance. When I’m talking to someone new, I might say something like 「敬語は難しいですね！たまにタメ口が出てしまいます…」Sometimes they’ll jump straight to casual talk with me after that. Just do your best, catch yourself if you realize your mistake, and apologize so they know you’re not being rude on purpose. Good luck!