How often are drug tests inconclusive/diluted?

So yesterday I took a drug test for a job I recently got. I know that I'll pass, I had to take one for a government job that I ended up not taking and passed, I haven't toked since mid January so it's well out of my system, or should be. I was hoping that I could just take it, hear back soon that I passed, and be able to (finally) toke again without worries, as they won't test me again and don't do random tests with this employer. Thing is, the HR lady who I was in touch with is out of work until next Monday, so even if I do pass I don't know that I'll hear back for a whole week. It's kind of driving me nuts because I don't start work for another couple weeks, and 4/20 is obviously Friday, and I have a whole stash I've been building up calling my name. I know I'm just being impatient, and that maybe I should just wait. I'm just really nervous that even though I know it isn't in my system, that it might somehow come up diluted or inconclusive for whatever reason and I'll have to retake it, and if that happens and I decide to smoke this week I'll be shit out of luck.

So my question really is how often do tests happen to come up diluted or inconclusive? Is it a common thing, or does it really only happen when the person drinks too much water or uses detox to try and get away with it? I don't even think I drank much water before taking it, just a couple cups of coffee and a couple croissants for breakfast. I feel like I'm just being paranoid, but I really don't want it to happen and already have smoked.

Any info or experiences would be awesome, thanks!

 

If, for example, I peed in a clean container today and refrigerated it, but had to retake it Monday, would it still be ok to use? As long as I got it up to a certain temp before putting it in I would assume