Always spellcheck your work before sending it to beta readers or especially before uploading it to publish. Be sure to do so after completing each edit as well. It’s really easy to get distracted and either not eliminate words or perhaps delete too many as you reword, streamline, and refine your sentences.
I don’t know about you, but my fingers have a mind of their own on the keyboard and don’t always do what my brain thinks. I will even catch myself from time to time typing a word phonetically for some reason.
In most cases, a grammar checker should find missing or misused words. But nothing beats a good proofreader or editor to make sure nothing was missed. And, believe me, it always is, no matter how many times you’ve been through it.