One of the theoretical solutions to an asteroid hurtling towards Earth is to nuke it so it breaks up into many small, harmless pieces. But that won't work if it can just reform in 2 hours.
Yes, if the nuke used is too small, the asteroid will break up into pieces that will fly apart so slowly that they'll just reform together due to their mutual gravity. How long will that take? Oh, 2 to 18 hours.
So yeah, it's good we realized this before trying to save ourselves with a nuke. We'd celebrate for a couple of hours before being really, really pissed. Yikes. [New Scientist]