.onInit must be on the top of the script to get rid of the internal banner. The internal banner is displayed when decompression has to be done before the installer is shown. This can happen when you're using solid compression and .onInit has to decompress files that don't belong to it from the solid block before it reaches the plug-ins/images/other files it needs. For example, the following script will trigger this banner.
To get around this, put .onInit before any other sections or instructions that may add content to the data block or use ReserveFile to make sure the files needed by .onInit are at the beginning of the data block.