Of course you might generate private key associated with address offline. In fact even with pen and paper alone if you know how
If you mean that you could generate an offline key that corresponds to an already existing address, then that is just astronomically unlikely (the address space is 2^160 which is just a vastly huge number). The only way this could happen is if you are using a seriously broken random number generator, or a brain wallet (just don't go there, you
will lose your coin unless you know exactly what you are doing).
And props to anyone who manages to convert a private key to an address using just pen and paper. Possible, yes, but the process is so mind-numbingly intricate that the chances of getting it right without making a mistake along the way are pretty tiny.
Just for LOLs, a link to my
very first post where I do indeed generate the private key to existing address 1EHNa6Q4Jz2uvNExL497mE43ikXhwF6kZm (its 1 by the way).