I'm launching a free in-browser file converter this Sunday. What conversion always breaks for you?

On Sunday (Aug 30) Softorino Online Converter goes live here on Product Hunt. It's a free converter for video, audio and GIFs that runs entirely in your browser, so files never leave your device. No sign-up, no ads, no watermarks. You can already try it at https://convert.softorino.com and before launch day I want to catch the annoying cases: what's the one conversion that always turns into a fight or sends you to a sketchy website? I'll make sure the worst offenders work on day one, and I'll be around answering everything.