Wow, haven't been on YE in a while. Glad I stumbled back today. I think this is a really good thread.
Here are some failures: some mine, some entrepreneurs' whom I worked with (sorry for the length, getting my YE fix all in one shot

) :
#1 (failed due to poor planning)
When I was in high school I had a "business" (sole proprietorship with a DBA registered with the state). --Side note: Sole proprietorships are NOT recommended by most business counsellors. I was still a minor at the time and not making much money so it probably was good enough in my case, maybe, but not recommended.-- I was offering design and printing services to local businesses along with a local community newspaper that I edited and delivered (sometimes with help from friends).
It was a good combination because the newsletter let me show off my business's design work and at the same time it attracted attention from local businesses that wanted to advertise in it. I did menus for restaurants, business cards/flyers/etc., custom t-shirts for a local school. It was going really well (I was even invited to talk at a youth business symposium - oh, the heights from which I have fallen

). The problem was I was getting SWAMPED with work. There were likely many more problems than that (managing the finances for example) but being overworked was the straw that broke my back. I gave it up to focus more on school. Looking back now I realise that I never really planned what I would do once the business was successful (the #1 business killer in my opinion), i.e. how to get all of the work I like doing done while doing 5 million other necessary parts of the business I don't like doing. I never planned when I would need extra help or how to use the extra help if I could secure it.
Failure-witnessing #1 (cash management problem)
I ended up spending most of my years in college in Japan (that is another story entirely, but this won't make sense without it). While in school, a friend introduced me to an American who had opened a music store in Japan (and was struggling). I introduced myself to him and told him that I would work for him for free if he would teach me how he started his business. Prior to the music store he had a medical equipment business servicing the US army bases in Japan. He used some of the profits from that to open his store (at this point he was still doing both businesses). Things were going great at first, really great. And then it happened. He had a substantial amount of money that he was due for the sale of some medical equipment but it ended up being held (in escrow I think, I don't know the exact details). This happened RIGHT before the grand opening of the store and prevented him from purchasing sufficient inventory. Long story short, the business (undercapitalised) went into a cycle of demise. I met the owner around the middle of that cycle. It was sad and at the same time inspiring because although the business was failing, and eventually had to be put down, he never let that break his spirit. I 'd like to think that my being there to help him at that time helped keep his spirits up a bit too. I learnt A LOT from him. It was a better education than four years of business school provided. I learnt from him, because even though he was failing he was very knowledgeable (so failure doesn't make you a failure!), and I learnt from his failure (I learnt what to avoid). A WARNING: learn cash management, bookkeeping is #1. If you are not careful an unexpected expense or other snag can kill your business. I've been reading a great book lately "The Small Business Survival Guide: How to Manage Your Cash, Profits, & Taxes" (You can probably find it or something similar at the library), I HIGHLY recommend it.
Failure-witnessing #2 (cause: legalities)
This one is a bit unique to starting a company as a foreigner in country. A friend of a friend started a financial services company in Tokyo (actually it was a branch of a company headquartered in Singapore that he convinced them to let him start in Tokyo). Anyway, he hired me and grabbed a few people from a competitor and things started rolling. It was great at first and we were profitable above what he had projected to the bosses in Singapore. Then came the legal problems. We are all foreigners which meant some visas had to be renewed and on top of that were problems with the legal formation of a branch office in Japan. Long story short things did not get done in time, my visa was set to expire and chances of the company sponsoring a renewal looked slim, I had to leave Japan. The branch failed but I learnt a lot from the experience (both what to and what not to do). Like the cliche says,
"Know before you go." Learn what legal hoops you need to jump through before you set out and before you start making promises it turns out you cannot keep, like sponsoring visas (for your employees' sakes). Even in Japan there is an organisation where you can get this information for free in English, so in an English speaking country like the US, UK, Australia one has no excuse for not knowing. In the US you have the SBA and S.C.O.R.E. and in the UK, Small Business Service (now "Enterprise Directorate").