1. Not storing secrets properly or using hardcoded secrets
2. Wild use of webviews/iframes sometimes easily propagates as XSS in phones
3. Incorrect client-side OAuth 2.0 configuration e.g. with schema-based redirect URLs.
4. Not supporting high-enough API versions, which may prevent some OS-related weaknesses
5. The list is actually very long. Just few top of my mind.
Fantastic answer thank you
Doing anything right on web is 10x harder and more complex. The problem is the browser, once you use it to deliver anything you have to buy into all of it’s bullshit. CORS, XSS, headers, caching. All that just goes away (outside your backend API, if you even need one) when you ship a native app