Sometimes, we fall into the trap of being human - there's certain things that we all just tend to do, and while they do make sense, usually, in an evolutionary context, it might lead to less than welcome outcomes in our modern world. Being human, and having these cognitive biases that we all have is, for instance, something that makes us vulnerable to con artists. Or to just acting stupidly all on our own.
One of the biases is the sunk cost fallacy - you've invested time, money, or both in something, and although reason tells you that it is a dead end and you should cut your losses, it is extremely hard to do so. I have definitely been a victim of that more than once! Confirmation bias is another very common one: favouring things that confirm what you already believe instead of staying objective and looking at your data in a more neutral way. Guilty, Your Honour.
Then there's also logical fallacies, such as using personal experience to dismiss statistics, or looking for patterns to fit a presumption (which sort of ties in with the confirmation bias). Recently, conversation between the Most Patient Husband of Them All and me has meandered to logical fallacies, and he has consequently presented me with a link to a very nice website, with lovely posters listing both biases and fallacies, which I now pass on to you for your delectation. You find them, and more, at www.yourfallacy.is and www.yourbias.is. Enjoy!