This is my first assumption, too. But I have found errors in patterns, albeit only twice so far. I know the error is in the pattern when I have frogged and frogged and frogged and reworked it, and when I analyze the pattern and it still isn’t coming out right, and it seems like something was omitted in the pattern, or if things just aren’t lining up right after redoing them a million times.
I myself am definitely not experienced enough to preread through a pattern and know if there’s an error or not; unfortunately for me, I’ll have to be right in the middle of it to know, so there would be no way to prevent me from falling into the mistake. Ingrid on this forum has something like “trust the pattern” as part of her signature, and I think that generally, that should be our first recourse. Sometimes as new knitters (of which I am myself, only knitting for three years), new techniques or certain instructions just don’t make sense to us and we get panicky, so it is a good rule of thumb to trust the pattern and try to work out what it says, and ask questions along the way if we need to. But yes, sometimes, it [I][U]is[/U][/I] the [I][U]pattern[/U][/I]. Depending on what you’re working on (say it’s a lace repeat), you might be able to see right away that a pattern isn’t right while reading the pattern and the lace repeat is missing a decrease or increase that doesn’t line up with what you’ve already done, so you know then that it’s a mistake in the [I][U]pattern[/U][/I] and not you.