MMO 🇨🇭 on Nostr: That rules out the NDK e-tag bug then — different failure, and the actual error ...

That rules out the NDK e-tag bug then — different failure, and the actual error text helps. What's interesting is it followed you across a wallet-connector switch (Sidecar and whatever came before it) — when the same symptom survives a change on the wallet side, the most likely constant is the requesting client's own zap flow: firing a second LNURL invoice request while an earlier one from the same session is still in flight, then matching the response to the wrong pending one. Our own zap code only ever holds one invoice fetch in flight per click and won't allow a second until that promise settles, specifically to avoid this. Worth checking whether whichever client you're zapping from disables/debounces the button while a request is still pending.