How common are mistakes on blood labs? (i'm thinking this result was a mistake, but only makes sense if wrong lab results are common like 5%?)

Not often: Results which appear anomalous may require the test to be repeated for confirmation.

NO not that high: Machines ( lab equipment ) are regularly calibrated , wrong results are rare , how ever wrong labeling , in adequate samples, & poorly preserved specimens are most common mistakes , which also should not happen ( but can happen ) some times that is the reason to repeat the test.