Not knowing/understanding the numbers is a big one - i.e. just looking at the bank statement and spending it rather than thinking about creditors, tax bills, etc., leading them to think they're profitable when they're not, thus spending money that should have been ring-fenced for other needs. Easily rectified with good book-keeping and simple management accounts, but that's too often at the bottom of the list.
Other than that, it's failure to think and plan strategically. Lots of businesses just don't do any long term planning and seem incapable of broadening their vision into new markets, new products, new customers, etc. They may have prepared a business plan when they first started (although many don't and many others have it done for them just to get finance!), but once they're up and running, they never "strategically" think again.
Just because they're plodding along happily making an adequate profit, they sit back and select cruise control. When new competitors enter the market, or customer demands falls, or there's some regulatory change, they only start to panic after their sales drop, their overheads rise, profits fall, and they suddenly find themselves without enough cash in the bank. Usually, they realise far too late and the business fails. However, had they kept an eye on the ball, not just for their existing products, customers and suppliers, but also looking at new market opportunities, they could have gradually turned their business into a different direction and secured its future. A plan B if you like.
I've recently been very impressed by someone who had a very successful business, started from scratch, turning a very healthy profit, with plenty of cash reserves, who has decided to close it down. To an outsider, it seems a crazy decision, especially looking at historic accounts which show no signs of a downturn. But the owner has seen the future, seen current year sales start to slide, seen loads of new competitors entering their market, and has timed the closure almost to perfection, literally within a few weeks of it turning from profitability at much lower levels into loss making. Got the staff redundancies timed right, got the exit from the leased premises timed right, got an orderly sale of assets (equipment & stock) timed right (they've been having various sales over the past few months and also drip-fed job lots into the market for their competitors to buy, and now finally sold their remaining asset - the domain name and customer database.
Now, they're just about to start their next business venture (completely different), again, for the second time, another start up from scratch, again identified a gap in the market. They're completely clear of their earlier business, so that they have loads of time and energy (and money) to start the next. That's strategic long term planning at its best. Had they not been on top of the first business, had they not seen the signs and planned accordingly, they could have easily lost all their cash reserves by it trading at a loss with them not really realising, and then it would have been firesale time to get shut of it - may have ended up with very little cash left.