I've done close to 200 interviews for Facebook. If you're coding in Python, then presumably you think it's your best language. If you aren't sure how to increment in your best language, it looks a little funny.
But just that: a little funny. It's certainly not a deal-breaker; we don't expect perfect syntax at all. For all I know you often switch between Python and JS in your day-to-day work and could easily make such a small error in a high pressure situation. That said, I'll notice it and take note. If one of four or five interviewers says "<name> incremented funnily" nobody will care. If every interviewer is saying "<name> doesn't seem very fluent in their chosen language" then the red flags start coming out.
But just that: a little funny. It's certainly not a deal-breaker; we don't expect perfect syntax at all. For all I know you often switch between Python and JS in your day-to-day work and could easily make such a small error in a high pressure situation. That said, I'll notice it and take note. If one of four or five interviewers says "<name> incremented funnily" nobody will care. If every interviewer is saying "<name> doesn't seem very fluent in their chosen language" then the red flags start coming out.