- Multiple formats (lud06/lud16) in the same fields (often the wrong one)

- Poor CORS support means you have to proxy address fetching, adding another point of failure

- You have to fetch, cache, and handle missing data for a user's kind 0 and address metadata

- A mixture of url parameters, json and bech32 make things exceptionally ugly

- Zap servers often don't give you good error responses

- Zap splits use positional, optional arguments. Omitting weight for some zap splits and not others is considered ok

- The mixture of msats and sats might mean you send 1000x or 1/1000th as much as you intended

- Wavlake uses a non-standard zap tag which doesn't include pubkey or hint

- There are lots of ways to collect an invoice: NWC, nip 07, copy/paste invoice

Just off the top of my head