What is my 3/8" concrete drill bit hitting in the basement wall and not penetrating when I try to mount a TV?
You could be dealing with a vertical cell filled with concrete or a vertical piece of rebar. If you are not using a hammer drill, try that first and see if it starts going deeper. If not, take a loot at the hammer drill bit and see if it looks like it's dulling. If it is, you will need to try a Carbide Rebar Drill Bit.