Sci-Fi and Fantasy: Keep It Consistent
Far and away one of the most common issues I see with science fiction and fantasy stories is an inability to keep to their own rules.
Both sci-fi and fantasy are defined by their fantastic elements - the technology and (typically, though not always) magic, respectively. They offer writers a space to go wild, inventing new and creative ways to bend the story. Unfortunately, it's very easy for an author to write themselves in circles with these elements. A savvy reader will notice that, two chapters ago, Wizard William couldn't cast the Spell of Ultimate Power because the ritual could only take place under a new moon, but when the ritual does successfully take place, it's described as being under a full moon. Sci-fi can get itself into similar trouble dealing with reversed polarities and such. Without a very close read, it's easy to miss these kinds of issues, and they can cause particularly nasty continuity snarls down the line
If you're going to go into detail on your magic or tech, the best way for an author to keep themselves out of trouble here is to create a master outline that describes the rules of his setting, separate from a plot outline. For example, you could include just how fast FTL travel is, whether or not cloaking fields bend light so they're also invisible to the naked eye or just mask a ship from sensors, exactly how many members are in the Galactic Senate and how they're chosen, etc. Once that document is finalized, do not change it. It should only be used for reference. It can be added to, if a new detail of the world comes up - you may not know, when you start, that planet Oorwagnh has seven moons, three of which are inhabited - but it cannot be altered. When sending the work to an editor or a beta reader, include this outline so they can catch any places you've deviated.
Another alternative is to simply never go into significant detail on such issues. If the hyperdrive is broken, do the readers really need to know that it's because the flux capacitor is shot? Probably not. Is it in character for the all-powerful wizard to get into a deep metaphysical conversation on how his spells function with the arrogant knight? Probably not. If you never put forward any rules, you can't break them - and most of the time, laying out such rules turns into awkward exposition dumps. Alastair Reynolds and George RR Martin are both really good about this. With few exceptions, the tech in Reynolds' books Just Works, unless it Stops Working, in which case "it'll take a few days to fix" is about as much detail as the characters go into. Magic in Martin's books is basically never explained outside of a couple of appropriately vague statements, such as "Only death may pay for life."
Whichever route you choose, stick to it.