So, your question seems to be, if it happened once, was it just this once?
A recount could answer that question, too, after this unfortunate occurrence:
"Snipes' office accidentally mixed a dozen rejected ballots in with 193 valid ballots".
Source: https://www.nbcmiami.com/news/local/Broward-Countys-Election-History-at-the-Helm-of-Brenda-Snipes-500187112.html .