Email is the most annoying one. There's an old API that some apps use that always uses Mail.app, regardless of what my "default email client" is set to. SUPER annoying as I only have my work email in Mail.app and it's usually for things like bug reports or news article feedback, you know, things I do on my free time which have nothing to do with work.

The bug report ones is especially annoying as those emails are often pre-populated with a bunch of data that isn't so easy to just copy into a new email in a different mail client, like support attachments, etc.