While we're continuously learning new things about the brain and how it works, it mostly remains a mysterious organ. It has our best interests at heart -- usually -- but when it comes to concepts such as memory, the brain has a habit of getting things almost right. Not that you can blame it, the poor mass of neurons has a lot to think about, never mind dealing with your constant need to recall fragments of your past.
The folks behind the YouTube channel Alltime10s have put together a list of reasons not to trust your memory (as though you have a choice in the matter, lest you tempt paranoia).
One phenomenon born of the internet age is the "Google Effect", where instead of remembering a piece of information, we instead bring up where said data can be found. What's the air-speed of an unladen swallow? You'd be caught out by a certain bridge keeper when the only answer you can proffer is "Let me check Wikipedia".
Another relatively recent discovery is "recall distortion"; much like constant photocopying (or saving an image over and over again as a JPEG), each time you remember an event, it is forever altered. Slowly but surely, that memory is changed into something measurably different to the original. How measurably? I suppose that depends on how good your memory is.
Check out the video for all the rest of the reasons your brain is not to be trusted (cue an evil laugh from inside your head).