Years ago, back when I was young and stupid, I was going through some turmoil in my life and ended up not filing my returns, and when it got to be like a year later and on the verge of being late with the next year's returns, I ended up packing up all the paperwork I could find for both years and dumped it all on some poor H&R Block guy's desk and begged him "please fix this". I was sure I was going to be sent to Leavenworth or something.
The upshot? After all penalties, etc., the Gov (Fed + State combined; forget the breakdown) ended up owing me a few hundred bucks.
Nowadays I use TurboTax/efile, and get my refunds direct deposited in 10 days max.