Most common is just wasting their time with low-ROI things such as meeting with a large number of low-value mentors. We try to keep startups focused on talking to customers and building product.

A crazy one I remember was telling a pre-product startup that they needed to focus on defining their brand.

"Hello, pre-product pre-revenue startup. You know what you should be working on right now? Your overseas expansion plan. The $COUNTRY market is too small to sustain the gigantic company which you will inevitably turn into. Here, let's spend two weeks working on PowerPoint decks."

(For avoidance of doubt: this wasn't an accelerator I have or had a formal relationship with, and -- thankfully -- won't be on the short list for almost anyone reading HN. For further avoidance of doubt: if you ever have coffee with me in Tokyo and ask for accelerator recommendations I am likely to observe that plane tickets are cheap.)

yes, it would be really helpful to get some examples of the counterproductive advice.