diff --git a/src/plm/core/lang/LangJava.java b/src/plm/core/lang/LangJava.java
index 03b1d23f9..5e38c32dd 100644
--- a/src/plm/core/lang/LangJava.java
+++ b/src/plm/core/lang/LangJava.java
@@ -546,7 +546,16 @@ public Iterable<JavaFileObject> list(Location location, String packageName, Set<
 				if (file.getKind() == Kind.CLASS && file.getName().startsWith(packageName))
 					files.add(file);
 			}
-			files.addAll(classLoader.files());
+			// Only return the compiled classes that actually live in the queried
+			// package. javac trusts that list(package=P) returns classes of P and
+			// derives their binary name as P + simpleName; returning every compiled
+			// class unconditionally made it mistake e.g. plm...SourceFile for
+			// scala.reflect.internal.util.SourceFile ("class file contains wrong
+			// class") and broke compilation.
+			for (JavaFileObject file : classLoader.files()) {
+				if (file.getName().startsWith(packageName))
+					files.add(file);
+			}
 		} else if (location == StandardLocation.SOURCE_PATH && kinds.contains(JavaFileObject.Kind.SOURCE)) {
 			for (JavaFileObject file : fileObjects.values()) {
 				if (file.getKind() == Kind.SOURCE && file.getName().startsWith(packageName))
diff --git a/src/plm/core/model/LessonRunner.java b/src/plm/core/model/LessonRunner.java
index f567d543a..ed3041e9d 100644
--- a/src/plm/core/model/LessonRunner.java
+++ b/src/plm/core/model/LessonRunner.java
@@ -103,12 +103,17 @@ public void run() {
 		runners.remove(this);
 	}
 
-	/** Stop all the threads that were already started. Harmful but who cares? */
-	@SuppressWarnings("deprecation")
+	/** Stop all the threads that were already started.
+	 *
+	 * Thread.stop() was used here historically, but it is deprecated for removal:
+	 * on recent JDKs its presence in this framework source -- which PLM recompiles
+	 * in process when running a Java exercise -- makes that compilation fail, so
+	 * no Java exercise can run. Replace it with Thread.interrupt(), a cooperative
+	 * request to stop. */
 	public void stopAll() {
 		while (runners.size()>0) {
 			Thread t = runners.remove(runners.size() - 1);
-			t.stop(); // harmful but who cares ?
+			t.interrupt();
 		}
 	}
 
diff --git a/src/plm/core/model/tracking/GitUtils.java b/src/plm/core/model/tracking/GitUtils.java
index 57fe3c749..dff97c10e 100644
--- a/src/plm/core/model/tracking/GitUtils.java
+++ b/src/plm/core/model/tracking/GitUtils.java
@@ -104,6 +104,7 @@ public void createInitialCommit() throws NoHeadException, NoMessageException, Un
 		git.commit().setMessage("Empty initial commit")
 			.setAuthor(new PersonIdent("John Doe", "john.doe@plm.net"))
 			.setCommitter(new PersonIdent("John Doe", "john.doe@plm.net"))
+			.setSign(false) // never sign PLM's internal progress-tracking commits
 			.call();
 	}
 	
@@ -159,6 +160,7 @@ else if(res.getMergeStatus() == MergeResult.MergeStatus.CONFLICTING) {
 				git.commit().setMessage("Manual merging")
 				.setAuthor(new PersonIdent("John Doe", "john.doe@plm.net"))
 				.setCommitter(new PersonIdent("John Doe", "john.doe@plm.net"))
+				.setSign(false) // never sign PLM's internal progress-tracking commits
 				.call();
 			}
 			else if(res.getMergeStatus() == MergeResult.MergeStatus.FAILED) {
@@ -339,6 +341,7 @@ public void commit(String msg) throws NoHeadException, NoMessageException, Unmer
 		this.git.commit().setMessage(msg)
 		.setAuthor(new PersonIdent("John Doe", "john.doe@plm.net"))
 		.setCommitter(new PersonIdent("John Doe", "john.doe@plm.net"))
+		.setSign(false) // never sign PLM's internal progress-tracking commits
 		.call();
 	}
 
diff --git a/src/plm/core/ui/LoggerPanel.java b/src/plm/core/ui/LoggerPanel.java
index 7f6a04d19..392f918aa 100644
--- a/src/plm/core/ui/LoggerPanel.java
+++ b/src/plm/core/ui/LoggerPanel.java
@@ -47,6 +47,11 @@ public void log(DiagnosticCollector<JavaFileObject> diagnostics) {
 		Pattern isJava6Pattern = Pattern.compile("major version 51 is newer than 50, the highest major version supported by this compiler");
 		
 		for (Diagnostic<? extends JavaFileObject> diagnostic : diagnostics.getDiagnostics()) {
+			// Only show real errors. Warnings and notes here come from PLM's own
+			// framework sources, which are recompiled in process together with the
+			// student code; they are not the student's concern and only add noise.
+			if (diagnostic.getKind() != Diagnostic.Kind.ERROR)
+				continue;
 			String source = diagnostic.getSource() == null ? "(null)" : diagnostic.getSource().getName();
 			String msg = diagnostic.getMessage(getLocale());