What causes an incorrect gender Ultrasound?

This question has been on my mind since my last gender scans which I had at 16 weeks, and 18 weeksn both confirmed girl with really good girl looking potty shots - I think.

So why do I keep hearing stories about people who were told one gender only to give birth to another or find out in a later ultrasound? in particular girl turns into boy? what causes this to happen? If you have lots of really good potty shots of Girly lines is there the chance that the penis was hiding, hasnt grown at 18 weeks?

These stories have me paronoid, and I feel like going and getting another gender scan in the next few weeks or am I just wasting my money?
here is my 18wk potty shot for reference.Attachment 1044

In some cases, the clitoris is so swollen it looks like a penis.

or you can have a misplaced cord that looks like something it isn't, or something that is assumed to be a cord but in fact is a willy

Congrats on your girl!

mine is safe to say girl though right? although I have only been told girl from my ultrasounds i think its because my sway wasnt the best I'm just indenial.
(I only did 5 days abstainence then BD 2 days in a row 2 days before ovulation. But diet was about 98% girl diet.) I was always convinced that this would make it a boy.