I'm constantly surprised by the number of people who make it really hard for you to help them. It has hurt some people as I and others don't try to do things for them any more.
Some of the common mistakes:
Disrespecting introductions. I've connected startups to other people and then they treat the other person badly.
Misunderstanding motivations. I've seen founders expect the intro to be something it isn't or the intro to do something unreasonable.
Not being prompt. An intro is made and the receiving person takes weeks to respond. It's insulting.
Not making it easy for the other side. If someone is doing you a favor, don't expect them to drive across town to help you. And so on.
Asking for more right away. I've sent candidates to a startup only to have them respond with "Great! Can you send me xyz person next?" Uh, no.
Oh, and there's a last one: Not being gracious.
As last anecdote about that: I referred a candidate to the startup. They hired him. The startup sent me a nice ($100ish) thank you gift. Of all the 100s of referrals I've made in the last 12 months, that one has always made me smile. I knew the startup valued my help.