Keeping an Index on a Subtle Difference in Illumina Chemistries

I like to pretend in this space that I catch all the little details of the different sequencing platforms.  Well, at least over time I try to do that.  But ego aside, that is often a mark not made.  A bit of a year ago I discovered that there's a small difference across the Illumina family that is completely separate from how clusters are generated (Bridge Amplification randomly arrayed or Exclusion Amplification in nanowells) or the wavelengths of light used in the fluorescence microscopy (now blue on the newest NextSeqs, with superresolution microscopy coming soon)  or 4 color vs. 2 color vs. 1-color (well, really staged 2-color) chemistry for the reversible terminators.  There's a subtle difference in how the second index is read.  I'm not spilling a deep secret: it's right out in plain sight within an Illumina technical document