<human>: Some background info. As a web developer I'm always getting asked to do work for free. Besides this, I'm sure a lot of you guys can relate to being the computer support guy because you "know computers", by friends and family. 

Another type of semi annoyance exists where friends think they have the greatest unique idea, and without any funding or coding skills of their own they propose a joint venture where you'll get paid based on commissions of sales. Yes ladies and gents, what I'm describing here is a classic startup scenario. Most of these fail, but sometimes a few work out, which means that I tend to lend an ear to every new idea that one of my friends come up with. 

Like today. So I go and say yes, I'm interested, and within 15 minutes the conversation goes something like "Welcome onboard Partner!" Then just 30 minutes later while taking a dump, I realise that I've made a huge mistake. 

Now I don't want to come off as unreliable or skittish, so now I need to let the person down nicely, while at the same time keeping some dignity. 

Have you ever been in this situation? What are some ways to deal with it amicably?

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Can you succinctly summarize the text in 1-3 sentences?
<bot>: Offerred to do coding for a web site and got roped in as a partner, now I think its a bad idea and want out. How to get off the hook while not seeming full of shit.