I had 4.1 generate a raw json object which looks perfectly correct. Even when I paste it back into ChatGPT and ask whether it’s a valid json, it says yes. But in reality, there’s an invisible \u200b breaking character at the end that wasted me over 2 hours of debugging because there’s literally no way to spot it visually. 
I’ve heard about “AI watermarking” being a thing but how is it helping anyone if even AI itself can barely spot it? This is a joke. I was only able to discover the character by using the playground (API) as ChatGPT didn’t see any issues.