I think the worse advice I ever got was to file a lawsuit.
When you're a startup there are few things that are as much of a distraction or an expense as pursuing a lawsuit. I'm not saying there aren't times when it's necessary but I think some lawyers are too eager and forget how small their clients are.
I would just recommend getting a second opinion before you ever decide to sue another company when you're a startup.
Lock myself and the whole team in a dark room for a year to build the product.
Luckily, we did the complete opposite. We released the prototype within 2 weeks and it's very obvious that we were on a totally wrong direction.
Someone told me I was better off spending time trying to game Google rather than doing more traditional marketing, being responsive to my customers, and developing and refining my products. Sadly I spent 2 years thinking that someone was right.
The worst advice I ever got given was to ask users instead of customers.
The worst advice I ever got was that in order to succeed as a manager, you have to be mean and nasty. Thankfully, I never took this advice because it was so against my character that it just felt wrong to chance into someone that I clearly was not and would never be.
Unless it's completely malicious, most advice is 'good' advice as it's just a product of the givers experience. Maybe the technique they preach really worked for them or in their industry, but not for you?
I guess you just need to understand their experience and unique situation, and work out which bits apply to your business and which way to avoid.
Easier said than done though!
Worst advice I ever got was: run business strategy decisions by a lawyer.
It is like asking for expert opinion on the authenticity of a Picasso painting from a blind man.
You will be more lucky finding aliens from another plant than really business savvy lawyer. They do exist...
worst advice given was "Don't do it".