Test:
call org.bson.io.Bits.readFully(new ByteArrayInputStream("some sample text".getBytes()), new byte[20], 0, 10);

Expected:
10 bytes are read.

Observed:
java.lang.IllegalArgumentException: Buffer is too small

The safety check
if (b.length - startOffset > length)

{ throw new IllegalArgumentException("Buffer is too small"); }

should test for "<".