] During the STS-87 mission, there was a change made on the
] external tank. Because of NASA's goal to use
] environmentally friendly products, a new method of
] "foaming" the external tank had been used for this
] mission and the STS-86 mission. It is suspected that
] large amounts of foam separated from the external tank
] and impacted the orbiter. This caused significant damage
] to the protective tiles of the orbiter. Foam cause damage
] to a ceramic tile?! That seems unlikely, however when
] that foam is combined with a flight velocity between
] speeds of MACH two to MACH four, it becomes a projectile
] with incredible damage potential.

I've also read that they could not dock with the ISS... Changing orbits on the fly is not, apparently, an easy thing to do. Makes sense if you think about it.

NASA article from 1997 describes tile damage from foam