Due to Microsoft change in 12/2011 classes with a lot of comments (and a lot of comment boxes on approval page) will experience this.
1. Navigate to the FA Web folder (most common location is "C:\Program Files\Blackbaud\FAWeb7")
2. Open the web.config file using Notepad
3. Add <add key="aspnet:MaxHttpCollectionKeys" value="1500" /> to <appSettings> section. May need to increase the value if more students are added to a class.