I have a 700x28 up front and a 700x23 on the back. It didn't come this way - I also shredded my front tire and the shop offered me the 700x28 one. I blame it on negligence from both parts. I always knew in the back of my mind that it wasn't safe to ride with a wider tire up front - but alas, it's been about 5 months so far... no accidents (attributed to that, anyhow). But, I think I'll definitely switch both tires out for new 700x23's just to be safe for next season.