Yup.

That's why we keep harping on proper and frequent testing. I test my water several times a week, and do a full test each weekend:
Yesterday,
19,200 gallons vinyl
78 Deg F.
FC: 6.0ppm
CC: 0
TC: 6.0ppm
pH: 7.5
TA: 90ppm
CYA: 30ppm
CH: Only test 2 or 3 times a summer

Week before:
79 Deg (little sun, lots of rain)
FC: 1.5
CC: 0
TC: 1.5
pH: 7.6
TA: 90
CYA: 30
CH: 70

So, even though the water was clear and perfect, I KNOW that for CYA=30, I should have at a minimum, FC=3.0.

I also know that 1 gallon of regular bleach(5.25%) adds 5.25ppm of FC to 10,000 gallons. My pool is virtually the same as 20,000 gallons, so 2 1/8ppm would be added--that put my FC over 3.5--back in the good range, BEFORE a problem began.

Later in the week, after tons and tons of rain, the FC dropped again, so 2 gallons of bleach went in late in the week, resulting in Sunday's numbers, which are about as good as I can get.