The guitar solo in Comfortably Numb is one of the most produced pieces of audio to ever appear on a record.
The solo was played by David Gilmour on a standard Gibson guitar thru a rack of audio effects, and recorded into tape. The tape was then played in the speaker system of a huge arena, the type you can fit 30,000 people into.
Microphones in the center of the arena picked up the resulting sound and recorded it into another tape. This tape was then combined in the studio with all the previously recorded material.
Some kinky way of saving some money on a reverb effect.