If you concatenate two files with self-invoking functions together that look like this:

File A:

(function(){...A...})()

File B:

(function(){...B...})()

File A+B:

(function(){...A...})()(function(){...B...})()

You have two statements without separator. This happens when you cat files together and then minify them.

Now the author of file B puts a semicolon in front:

File B2:

;(function(){...B2...})()

And you'll get a working script:

(function(){...A...})();(function(){...B2...})()