Some people come to Pennsic with little knowledge of the Middle Ages and even less interest in learning about it. They have heard that Pennsic is a great place to drink and carouse. These people are annoying to the rest of us who care about what the SCA really tries to do.
Some people come to Pennsic knowing, or thinking they know, everything about the Middle Ages and insisting that everyone--even new people--be perfectly authentic at all times. These people are annoying to the rest of us who do the best we can but mainly want to have fun since this is, after all, just a hobby.
There is a happy middle ground between these two extremes, and most people will do well to pick a point in the middle according to their own preferences and resources.