What is wrong with FileInputStream.read(byte[])?

In response to my answer to a file-reading question, a commenter stated that FileInputStream.read(byte[]) is "not guaranteed to fill the buffer."

File file = /* ... */  
long len = file.length();
byte[] buffer = new byte[(int)len];
FileInputStream in = new FileInputStream(file);
in.read(buffer);

(The code assumes that the file length does not exceed 2GB)

Apart from an IOException, what could cause the read method to not retrieve the entire file contents?

EDIT:

The idea of the code (and the goal of the OP of the question I answered) is to read the entire file into a chunk of memory in one swoop, that's why buffer_size = file_size.