Studies have consistently shown that holding all else equal, subjects will predict positive outcomes to be more likely than negative outcomes. See positive outcome bias.
Prominent examples of wishful thinking include:
Some atheists argue that much of theology, particularly arguments for the existence of God, is based on wishful thinking because it takes the desired outcome (that a god or gods exist) and tries to prove it on the basis of a premise through reasoning which can be analysed as fallacious, but which may nevertheless be wished "true" in the mind of the believer. Some theologians argue that it is actually atheism which is the product of wishful thinking, in that atheists may not want to believe in any gods or may not want there to be any gods. Both of these arguments would better be described as confirmation bias. Since one rarely, if ever, finds an argument written or spoken as described above ("I wish it to be true, therefore it is true"), the charge of "wishful thinking" itself can be a form of circumstantial ad hominem argument, even a Bulverism.
Wishful thinking may cause blindness to unintended consequences.
Related fallacies are the Negative proof and Argument from ignorance fallacies ("It hasn't been proven false, so it must be true." and vice versa). For instance, a believer in UFOs may accept that most UFO photos are faked, but claim that the ones that haven't been debunked must be considered genuine.