I told my partner I was fluent in French on our first date. Five years later, we're moving to Lyon for his job. I've spent £2,000 on intensive tutoring in the last three weeks. I'm currently at 'The cat is under the table' level. I am terrified.
I had the opposite experience. I knew my boss socially, and he knew I could speak passable French, so he called me into a meeting, without notice, as an interpreter in a highly technical discussion for which I knew none of the specialist vocabulary. Fortunately, in computing, as I rapidly discovered, French mostly uses mildly Frenchified versions of English terms, so a Raid controller is "un controller Raid", where "Raid" is pronounced like the French word "raide". Somehow, I muddled through and we had a productive discussion. It was hard work, though!