The groovy.ico file in the distributions (source zip, binary zip and also Windows installer) is broken.
The one from https://fisheye.codehaus.org/browse/~raw,r=9382/groovy/trunk/groovy/groovy-core/src/tools/org/codehaus/groovy/tools/groovy.ico is fine though.
Comparing a hexdump of both files it seems there are Unicode replace characters inserted.
I guess you use a <copy> task with nested <filterset> or with some implicit filter from a stand-alone <filter> task to copy that file. This is not according to the <copy> tasks documentation, which states that filter may only be used with text files and will destroy binary files.
Note: If you employ filters in your copy operation, you should limit the copy to text files. Binary files will be corrupted by the copy operation. This applies whether the filters are implicitly defined by the filter task or explicitly provided to the copy operation as filtersets. See encoding note.