Volatile is useful for telling compilers not to stick  data in registers, which is helpful for multi-threaded situations with shared memory ... sometimes :)

If I were to write a semaphore or another synchronization method using shared memory on unix I'd probably type volatile and then pray :).

It often feels more of a faith issue than a knowledge issue though :)

Const is ok, sometimes, in some languages [C++].  It's notoriously difficult to add const interfaacs to C++ classes as an afterthought.  You have to go in "full const" or none at all.  Adding it afterwards causes a great deal of pain tracking down all the places you needed const when you didn't put it there. 

It's like playing pick up sticks with your buttcheeks. - [I think] the late John Candy.