That is a tough question to answer because everyone makes mistakes, sometimes it is the server's, sometimes the cook's, and sometimes the expo's, and sometimes it is the customer's fault. I wouldn't say that one person tends to make the mistake more often than others though. If it is the server's fault it is usually because they wrote the order down wrong, put it into the computer wrong, or forgot to ask the guest if they wanted to modify it. If it is the cook's fault it is usually because they read the ticket wrong or were confused about a modification but never got it clarified. If it is the expo's fault it is usually because they didn't read the ticket right, accidentally sent out a dish to the wrong table, or didn't check all the modifications. If it is the guest's fault it is usually because they forgot to make a modification until the food was basically done or already at the table and they sent it back.
What most people don't realize, is that there is a lot going on back in the kitchen. Your table might have only ordered two entrees, but there are several components that go into your entrees, usually coming from more than one cook, and each cook is working on food for the entire restaurant. Food is supposed to always be out in under 15 minutes, so people are working fast to try and get everything done. So, things happen and sometimes people read things wrong, don't remember a modification that was told after it was rang in, or they grab the wrong thing. But, there are few mistakes that can't easily be fixed.