It's unusual for a Shuttle flight not to have some popped tiles. I remember the first flight of Columbia (STS-1) when there were many popped tiles (mostly on the OMS pods). For STS-1, NASA waited for Columbia to overfly Hawaii where the USAF has a satellite inspection telescope. It was determined that none of the popped tiles were "killer tiles" and Columbia was permitted to land. There were instances in the past when a popped tile suprised people, e.g. the aluminum underneath had actually melted away. Something that has always worried people was the possibility of a "zipper effect". For this case a tile pops off that is not considered a killer tile. However the tile next to it pops off, then the next and so on until the chain reaction reaches a killer tile. I suspect this was the mechanism that killed Columbia.

Gary