Conventional wisdom for optics, despite their phenomenal track record of performance (at least on the high-end stuff) has dictated that a combat carbine should include a set of back-up iron sights. The operating theory is that if/when your main optic goes down, one can easily deploy the iron sights and resume aimed, accurate fire.
This is all well and good (and has come in handy for me at a 3-gun match when my main optic pooped itself), but few know that your main optic can spoil your BUIS sight picture. Simply put, your optic distorts light (as a function of being an optic), which can, in turn, distort your front sight when viewing through the optic. As such, if you zeroed without your optic in view, your zero can be off when viewed through the optic.
As a general rule, this is relatively minor with absolute co-witness, but those using lower 1/3 co-witness should pay special close attention. Almost all optics like the popular Aimpoint and various knock-offs used curved lenses to reflect the dot back to the shooter’s eye from the projector. When looking through the optic, this is not an issue, as the design accounts for the curved lenses to the projector.
But, the rear sight is out of position relative to the projector and when the sight is near the bottom of the curved lens, the curvature presents the most distortion of the light coming into the lens, which can move the front sight relative to its original non-optic-passthrough position, resulting in changed zero.
As such, if one intends to shoot through their optic with BUIS, make sure to zero with the optic in the plane to account for this. Or, have your optic on a Q/D mount so when it fails, it comes off the gun in a hurry.
KitBadger shows the phenomenon with a Trijicon SRO: