One of the most difficult things to find when you’re editing your own work is missing words. Sometimes even editors fail to catch them, but not an alert reader. Once in a while a grammar checker might, but not always. These are usually not quite as bad as typos which stand out like the proverbial sore thumb, but close. They tend to jolt the reader out of the story, even for a nanosecond, or sometimes longer if it makes the sentence difficult to understand.
When reading over your manuscript during your final draft, do so slowly enough to note each word is indeed written as opposed to assumed. Reading it aloud can help. If you’re fast on the keyboard, you may be even more likely to leave words out because regardless of how fast your fingers are, you brain is moving faster.
If you’re ever beta reading for someone and find missing words, be sure to tell them. The author will be very appreciative!