diff --git a/core/src/main/java/hudson/FilePath.java b/core/src/main/java/hudson/FilePath.java
index de175441bce8..992c9f49b379 100644
--- a/core/src/main/java/hudson/FilePath.java
+++ b/core/src/main/java/hudson/FilePath.java
@@ -14,6 +14,7 @@
 import hudson.util.FormFieldValidator;
 import hudson.util.IOException2;
 import hudson.util.StreamResource;
+import hudson.util.HeadBufferingStream;
 import org.apache.tools.ant.BuildException;
 import org.apache.tools.ant.DirectoryScanner;
 import org.apache.tools.ant.Project;
@@ -990,10 +991,17 @@ private Integer writeToTar(File baseDir, String fileMask, String excludes, Pipe
     /**
      * Reads from a tar stream and stores obtained files to the base dir.
      */
-    private static void readFromTar(String name, File baseDir, InputStream in) throws IOException {
+    private static void readFromTar(String name, File baseDir, InputStream _in) throws IOException {
         Untar untar = new Untar();
         untar.setProject(new Project());
-        untar.add(new StreamResource(name,new BufferedInputStream(new GZIPInputStream(in))));
+        HeadBufferingStream in = new HeadBufferingStream(_in,SIDE_BUFFER_SIZE);
+        try {
+            untar.add(new StreamResource(name,new BufferedInputStream(new GZIPInputStream(in))));
+        } catch (IOException e) {
+            // various people reported "java.io.IOException: Not in GZIP format" here, so diagnose this problem better
+            in.fillSide();
+            throw new IOException2(e.getMessage()+"\nstream="+Util.toHexString(in.getSideBuffer()),e);
+        }
         untar.setDest(baseDir);
         try {
             untar.execute();
@@ -1187,6 +1195,8 @@ private void readObject(ObjectInputStream ois) throws IOException, ClassNotFound
 
     private static final long serialVersionUID = 1L;
 
+    public static int SIDE_BUFFER_SIZE = 1024;
+
     /**
      * Adapts {@link FileCallable} to {@link Callable}.
      */
diff --git a/core/src/main/java/hudson/util/HeadBufferingStream.java b/core/src/main/java/hudson/util/HeadBufferingStream.java
new file mode 100644
index 000000000000..c63498cff6dd
--- /dev/null
+++ b/core/src/main/java/hudson/util/HeadBufferingStream.java
@@ -0,0 +1,69 @@
+package hudson.util;
+
+import org.apache.commons.io.output.ByteArrayOutputStream;
+
+import java.io.FilterInputStream;
+import java.io.InputStream;
+import java.io.IOException;
+
+/**
+ * {@link FilterInputStream} that buffers the first N bytes to a byte array on the side.
+ * This byte array can be then accessed later.
+ *
+ * <p>
+ * Useful for sniffing the content of the stream after the error is discovered.
+ *
+ * @author Kohsuke Kawaguchi
+ */
+public class HeadBufferingStream extends FilterInputStream {
+    private final ByteArrayOutputStream side;
+    private final int sideBufferSize;
+
+    public HeadBufferingStream(InputStream in, int sideBufferSize) {
+        super(in);
+        this.sideBufferSize = sideBufferSize;
+        this.side = new ByteArrayOutputStream(sideBufferSize);
+    }
+
+    public int read() throws IOException {
+        int i = in.read();
+        if(i>=0 && space()>0)
+            side.write(i);
+        return i;
+    }
+
+    public int read(byte b[], int off, int len) throws IOException {
+        int r = in.read(b, off, len);
+        if(r>0) {
+            int sp = space();
+            if(sp>0)
+                side.write(b,off,Math.min(r, sp));
+        }
+        return r;
+    }
+
+    /**
+     * Available space in the {@link #side} buffer.
+     */
+    private int space() {
+        return sideBufferSize-side.size();
+    }
+
+    /**
+     * Read until we fill up the side buffer.
+     */
+    public void fillSide() throws IOException {
+        byte[] buf = new byte[space()];
+        while(space()>0) {
+            if(read(buf)<0)
+                return;
+        }
+    }
+
+    /**
+     * Gets the side buffer content.
+     */
+    public byte[] getSideBuffer() {
+        return side.toByteArray();
+    }
+}