1. Publishing the Dall-E papers and pre-trained CLIP weights. This inspired Stable Diffusion/MidJourney, reducing the amount of time OpenAI had for their Dall-E service to get established by maybe a year. During that time, they could've gained long-term customers, generated some revenue, and established partnerships with Adobe and other graphics software makers. Now they're second string.
2. Publishing the GPT-3 paper and releasing ChatGPT for free online. Instead, they should've improved it by adding things like references and released it already integrated in Bing/Word/Cortana. This would've been more valuable to Microsoft. Now, Google has time to catch up and have their own model in their search engine not long after Bing if they do it right. And Anthropic is working on a chat model and will have a good one as well soon.
One possible counterargument could be that these haphazard releases allowed OpenAI to gain mindshare among researchers. But this is much more vague and speculative.