The following test case terminates with an assertion failure in Merlin build 46 (and it completes successfully in build 45):

import java.io.*;

public class TestStream {
    public static void main(String args[]) {
        try {
InputStreamReader r = new InputStreamReader(
new ByteArrayInputStream(new byte[] { 'X' }));
char[] c = new char[1];
r.read(c);
System.out.println(c[0]);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

Here is the failure in build 46:

Exception in thread "main" java.lang.Error
        at java.io.InputStreamReader$CharsetFiller.fill(InputStreamReader.java:353)
        at java.io.InputStreamReader.read(InputStreamReader.java:478)
        at java.io.Reader.read(Reader.java:103)
        at TestStream.main(TestStream.java:9)

The problem seems to be this assertion in line 353 of InputStreamReader:

        int fill(char[] cbuf, int off, int end ) throws IOException {
            if (!(end - off > 1)) throw new Error(); // ## assert

which fails if (end - off) == 1, which seems to be a legitimate input as far as I can tell. Shouldn't the assertion be just

if (!(end > off))

?