I am a native speaker of English, but I make (and notice others make) English mistakes all of the time; I change negative to positive and vice versa, I switch "he" and "she", I make spoonerisms, I leave 's' off of third person singular, and so on. Mistakes just happen. I hear native speakers make mistakes in my second language as well. One difficulty for me in becoming an advanced speaker of this language was in getting over the occasional error. Eventually you realize that natives make mistakes, too, and one's time is better focused on enriching one's manner of expression instead of worrying about spoonerisms (especially if you're the type who makes lots in your native tongue).
I think it would be helpful to know what the average speaking error rates are among native speakers, so as not to hold oneself to the impossible standard of 0% error. Are there any studies showing average error rates for native speakers of a language? I know "error" can be broadly interpreted, but take my examples above to be approximately what I'm considering an "error".