More likely than you'd expect. Rumors of blocks being invalid after 20 blocks is not unheard of.
a block does not get paid into the pools wallet until 120 confirmations, most pools will not payout before this is reached.
some pools pay earlier, it means the pool operator is taking a risk that the block will go invalid before 120 confirms
You say risk, but ... what is the likelihood of that actually happening
after - say - five blocks ?