Egregious

Etymology

From Latin "ēgregius," "distinguished, standing out from the flock," from "ē" (a form of "ex," "out of") and "grex" ("flock, herd"). It first meant remarkably good — set apart above the crowd. In the 16th century ironic use flipped it, and the "remarkably bad" sense hardened into the only one that survives.

adjective · /ih-GREE-jus/
Outstandingly bad; conspicuous in its awfulness.
What it names
A mistake or offense so glaring it stands out from ordinary failings — an egregious error is not just wrong but remarkably, memorably wrong.
Example
The report contained an egregious error: it had confused millions with billions throughout.
Related idea
Near flagrant, with a difference of focus: flagrant stresses that a wrong is done openly and defiantly, while egregious stresses the sheer scale of how bad it is.