Twice in the studio version, there's this phrase (previously, I'd thought it was trumpet, but now I think it's French horn):
In the live version, Rod plays this phrase on Pianet, but he changes one note so that it's now:
According to the Zombie Heaven liner notes, the studio version of "Goin' out of My Head" was recorded 23 October 1966. The Decca Stereo Anthology liner notes confirm this, adding that the orchestral overdubs were "probably recorded at a later date." I found conflicting dates for the live recording, though: the Zombie Heaven liner notes say 1 November 1966, but The BBC Radio Sessions liner notes say 29 October. According to Zombie Heaven, however, the Zombies were doing appearances in Paris on 29 and 30 October, which Colin alludes to in the interview with Brian Matthew that precedes the performance: "We just had a very hectic weekend in France and Belgium, last weekend." Colin also introduces the song by saying, "We're gonna record a song called 'I Think I'm Going out of My Head,' which was originally recorded by Little Anthony and the Imperials." This comment would suggest that this live recording actually predates the studio version, although the dates provided don't align with this.
In examining the studio version of the song more closely, I found a couple more features to note.
During the line "I can't think of anything but you," the vocal harmonies come in and coincide with the "you" in the lead vocal. In a way, they literally underscore the word, highlighting its importance for the narrator.
I finally did some more work on transcribing the bass part (after a lapse of about two years). Under the line "I must think of a way into your heart," it's something like this:
The first measure consists of two triplets (this rhythm is also sung in the vocal part and played in the drum and trombone parts). Because there are six notes (really a rest and five notes) forced into the space where there are usually only four, there's something of a sense of the narrator's determination ("I must"). Alternatively, this uncommon rhythm simply draws attention to the narrator's declaration.