Gaaahh! I hate early symptoms!.... it's harder by far to pin something down until it progresses worse...X makes a good point, if it was only a handful of older leaves, maybe deeper inside the canopy, on a few plants, I wouldn't sweat it,... but it's on several, happened very fast, and the worst plant has a pretty wide span of affected leaves..... I looks like a defc., not Ca though, more like Mg, but with all the dosing lately and TLO,...naahhh.... Jm, X, could the pendulum have swung to far the other way while treating for Ca and developed a toxicity issue with K, or even Mg? The only other thing I would guess at is a Ph lockout, alkaline, and heavier metals like Fe, Mn, Cu, Zn,....