One of the worst mistakes a novice author can make - which, conversely, is one of the unique hazards of Speculative Fiction - is to include a Mary Sue character in their writing. A Mary Sue (the term is actually not gender specific) is a character who can do anything with no explanation where these skills come from. In its more respectable, traditional, form, they are known as a JOAT - a Jack Of All Trades. But if a JOAT is to be a believable character, their abilities will be finite and compartmented into one specific field. A JOAT can fix any system on a space ship, but knows nothing about medicine or cooking. Moreover, while a JOAT has all kinds of 'book-learning' and experience, his/her skills will be finite, and a believable character will have some offsetting disability, such as a missing arm or a crippling mental block. This will make the JOAT's struggle all that more dramatic, which is always a good thing. He/she can repair the ship's navigation system (given the parts) but can't do much for the melted-down reactor, and is deathly afraid of the ship's cat.

A Mary Sue, on the other hand, can fix anything, solve any riddle, answer any question, fight with any weapon, fly any space craft, deal with any archane power - but nothing is said about where he/she obtained these skills. Indeed, the story line disputes any reasonable explanation for these miraculous abilities, granting top flight talent to the novice who can match those of even the most experienced old hands. It just happens that the character can conveniently rise to any challenge. Imagine that.

This is the essence of bad hack writing. Almost as bad is trying to explain this miraculous ability with a Deus Ex Machina such as the mind programming in The Matrix, or mind-melding ala Mister Spock. These can be used, but you are treading on thin ice. This is also a cheap cop-out. As discussed in The Origin Story your character should go on a quest of self-discovery to develop those skills, which can generate interesting story lines in themselves.

One of the essences of good writing is to build a character's growth arc, ideally through long and perilous trial and error. A Mary Sue, knowing it all already, can offer nothing but mild surprise when he/she 'discovers' a critical talent at just the right moment. Without that character growth, you really don't have a character. And without characters, you really don't have a story.