96/365 - Now one the most important testings, this one really has to be planed ahead so you avoid large amount of bugs. Don’t be delusional thinking that errors in this phase can be avoided completely.
Being honest, never expect 0 errors ever, regardless the stage of your project.
The interface test aims to find possible connection errors in the application server and database server interface, and/or the web server and the application server interface.
A pretty rough definition for interface would be a connection between two constituents. Any software interacting with the application can be considered a component here.
And that is what makes this stage so important. In my experience, dealing with APIs (even though necessary for many projects) is the easiest way to get tones of bugs.
Check if the sever is executing as supposed to, also if errors are handled without compromising the application. Another thing that I like to do while running this test is to check the user-friendliness of the whole project.