Continuity mistake: Phelps takes the tape from the recorder's right-hand spindle. When he opens the acid vat, we can see the tape he just removed back on the recorder - but on the left spindle. The tape he then drops into the acid doesn't remotely resemble the one he pulled, and when he drops it, the recorder's left spindle, where the full tape was in the last shot and where the empty reel should be, now has nothing on it at all.
Trivia: Apollo rewinds a reel-to-reel tape recorder that has just recorded him admitting to illegal activities. As he does, it makes the expected chipmunk noise. When slowed and reversed, the tape is of a speech given in episode #4 from the 3rd season. You can hear these recordings at: http://www.litb.com/recording1.mp3 and http://www.litb.com/recording2.mp3.