Well, usually it's the startup phase. If I get into the rhythm of writing, I can keep writing and writing. My high fantasy epic was a true epic in the sense of being egregiously long (about 60-80 chapters, and about 386k words. For reference, a typical novel's like 50k at most, with a typical short story as brief at 5000 words or less). But if I don't have an idea that I'm psyched about, it usually stall out and it takes me months to update chapters.
Thinking up a new story is tough for me, but I don't generally get blocked (mainly because I push through it). What I do find is that I type quickly and tend to make typos that can't be caught by spellcheck (they're usually homophonic or similar letter errors because I'm sounding out what I want to write in my head). Like where I know the difference between wear, where, were, and we're when I'm typing and paying attention, but not if I'm tired or get into a fast rhythm.
There was this movie about a writer like that. He was searching for the perfect words to write his perfect story. But he was blocked and stuck on page 4 or so (I have a feeling it was Throw Momma from the Train). You're going to make errors. This is what editors are for. But if you break your rhythm, the story doesn't flow.
Better to have errors (hell, I have continuity errors for things that I repeated several times, then decided to change) than to stall and not finish. Perfectionism is at least as bad as procrastination.
