It seems to happen to every musical artist at some point or another. That particular album or song that's distinctly below par or lacking in inspiration. Very few musicians seem immune to this.
I'll firstly nominate Queen's 1982 album 'Hot Space'.
This was the album where Queen, especially a couple of the members of the band, took their audience's willingness to follow the group's experimentation down the years and pushed it a little too far. Queen's music had developed like a metaphorical 'rubber band' up to this point. Queen fans were happy to see the group try out different genres of music as long as they retained an element of 'rock' at their core, hence the rubber band metaphor. On 'Hot Space', a largely synth and disco based exercise, that rubber band was stretched too far and snapped. You've got 'Under Pressure' on there but little else of note. This was Queen's lowest selling album by far at that point.
What other music is there out there by legendary artists that is sub-par for whatever reason? I know that I personally waffle on about Classic Rock and Metal artists but this thread is open to music of all genres.