Rode over something with the back 700c wheel (Shimano WH-550) commuting a couple of weeks ago, not sure what it was, but it was hard and sharp and made a crunching noise as it sliced through my tire punched a hole in my rim. I was going pretty fast down a hill at the time. Never was in any danger, but I was surprised at the damage … a gaping 1/4 inch hole in the rim (see pic its located between the valve-stem hole and a spoke-nipple hole). Be careful out there.
The rim
The tire