DAVE Easter egg
From Halopedia, the Halo wiki
The DAVE Easter egg is an Easter egg found in the Halo 3 Beta and in Halo: Reach. Viewing the Easter egg in Halo: The Master Chief Collection will unlock the I'm Sorry, Dave achievement.
The Easter egg can be found on the Beta Mark VI BIOS and the Reach Mark V BIOS in any gametype. When a non-scoped weapon is zoomed-in, a line of binary code is visible in the lower-left corner. In both games the numbers read 01000100011000010111011001100101. When translated from ASCII eight-bit binary code, the code reads "Dave", a reference to Bungie interface designer Dave Candland.