Pregnant after vasectomy??

I gave birth to our third and final baby 6/23/2017, my Husband had a vasectomy in September 2017. He wasn’t told the correct procedure to bring in his sperm sample, he ended up going somewhere first and left the sample inside where it was hot for a little over an hour. It looked like water but he brought it in anyway, they called him that same day saying that the count was zero and that he wouldn’t have to come back. Fast forward to now, I’m two weeks late but all of the tests are negative. What’s going on?! Has anyone gotten pregnant after vasectomy?