A minor issue, really, but one that's slightly annoying.
The way I noticed it, Spotify has added new advertisements on the "What's New" tab, which play when you first logon, but also when you go back to that page.
The ads in between songs are understandable and tolerable; they wait until I'm finished with a song before playing. But these will run on top of whatever you're streaming. I think there should be some way to make sure both the ad and the song aren't playing at once; at the very least, let one get muted while the other plays.