If you forgot the 30 minute hops, the beer will have lost some bittering and not flavor hops. The only issue is that the beer may end up being too sweet due to the lack of bittering. It really depends on how many AAUs were in those "lost" bittering hops. Dryhopping won't help because the 30 minute hops aren't giving you aroma and flavor but instead bitterness.
If the beer is too sweet after it's finished, you may be able to help correct it by boiling up some hops in a little water to extract some bittering and add that to the beer. But I'd wait and see before trying to fix something. Since 30 minute hops aren't fully isomerized, you may only be short a very few IBUs.