It only causes problems for me if I have a client who owe's enough to compute a penalty. And those folks aren't mailing the check until April 14 in any case, so that file just goes in a pending stack to be filed when the form is available.

This happens every year, different form, but I will have a (very) few returns that have to go in a "waiting for forms pile".

It's not a big deal after 40+ years. (BTW that pile is right next to the "Waiting for Clients to find the last 3 pieces of information pile", which is much more of a PITA than the "Waiting for forms" pile) you just learn to deal with things.