Overdraft fees can defintley fuck your cashflow up.
Let me tell you a story, when I first started I eWhored for like $2,000 over a period of a few months. I thought, "Great, now I can invest in PPC campaigns." It got a little more expensive than I estimated and I overdrafted by about $20 on a $500. Not only that, but I had "Overdraft protection" supposedly. But of course, small text int he contract fucked me up.
Not only did I get charged $30 every 24 hours I was overdrafted, but I was also charged $30 everytime the advertising money tried to pull money out of my account. Within 48 hours, I ended up with about $700 in overdraft charges. Luckily, the ads converted and I eventually ended up being able to pay it back. I closed my account shortly after that citing to them, "Your policies are meant to fuck your customers right in the ass." The manager laughed, and escorted me out. This is one of the major banks that recieved BILLIONS in bailout money. Go figure.
I'm one of the lucky ones. I transferred with a few hundred dollars to a bank I'm very happy at now. I can't even post all the stuff they do/have done for me. For instance, I came on an overdraft and instead of just charging me a shitload, the new (good) bank called and asked if I wanted to take out a $500 loan at 7%/annual to cover this. I agreed, came in, signed paper work and had it payed off the month after. The bank made a little (~$20) and saved me alot.