Sat drank a normal amount of fluids and got another test to take Sun morning. AND FAILED (went right in the cup darker yellow) And by "failed" I mean the line was so light that if I was a parent testing my kids I would have sent it off to the lab to confirm.
Any thoughts on why I was able to pass a home test on Friday then fail the same test on Sunday.
You have misinterpreted the results of your Sunday test. Youve stated that the line "was so light" etc, etc; - it was still a negative. In order for the result to be non-negative, the "T" window must remain completely blank.

Quote Originally Posted by sunburstkid
You said that you passed the test on Friday at 9pm. You are much more likely to pass a urine test at the end of the day than with your first void of the day. Also, it is common to pass one day and then fail the next since the metabolites are released every day and some fats may have a greater concentration than others.

I've been clean for 41 days now. Up until day 21 I routinely failed at home tests. From day 21 until day 28 I passed, but on day 29 I failed. Since then I've passed every morning. I think I was a bit dehydrated on day 29 leading to a more concentrated sample. However, it goes to show that just because you can pass a test does not mean you are 100% detoxed. Staying hydrated, testing later in the day, and collecting mid-stream all contribute towards making sure your sample is clean.
Anytime you take a home test, youd always use the first-mornings void when THCA concentrations would be at their highest. Taking the test at the latter parts of the day skews the results due to the fluids that are consumed during the normal course of the day. The fluid consumption dilutes the THCA concentration in the bladder to the point where the sample registers below the cutoff thresholds of the home test (that is providing that the THCA concentration is not too high).
The reason why a person tests negative at night, but fails the first-morning test is because the dilution phenomena is only temporary - once fluids ingested had all been voided out, the bladder is replenished with fresh THCA-laden urine by the kidneys, and thus the donor would resume testing positive for THC.

By testing your first-morning void, you are testing yourself under the "worst-case scenario" - again when THCA concentrations would be at their highest. So if your home test is a negative under that "worst-case scenario" (regardless of line intensity) , you'l pass a U/A given at any time.