Consider the following from One's Wikipedia article:
The band returned to Dublin in 1991 to record at the "Elsinore" mansion on the Dalkey coastline. The band continued to work on the song there, adding various overdubs, but not finding a mix they were satisfied with. The Edge thought that they had the foundation for the song, but that it needed "foreground". Eno interceded, explaining to the group that "One" was among the sessions' tracks in which "The song has gone, whatever it is you liked about this song is not there anymore", and that the track had "disappeared under layers of overdubs". He created his own mix, which gave the band a better idea of an arrangement they liked. Eno wanted the band to remove the melancholy elements of the song and persuaded them to remove the acoustic guitar from the song. He also worked with Lanois and The Edge to "undermine the 'too beautiful' feeling", which is why they added the "crying guitar parts that have an aggression to them".
Flood, the sessions' engineer, was unconvinced by the song's mix, saying he "was the nagging doubter. I always felt it was a bit straight, until we did the final mix." The final mix was completed at Windmill Lane Studios in September 1991 on the last night of the album's recording sessions,when some last minute additions were made. Bono did not like a line in the vocals and spent most of the day re-recording it. Later, after the song's mix had just been completed by the production team, The Edge came up with a guitar part he wanted to add to the song's end near the lyric "Love is a temple". After convincing the production team to allow the addition, The Edge played the part once and had it mixed in ten minutes later.