Index: /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java
===================================================================
--- /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 7297)
+++ /trunk/src/org/openstreetmap/josm/data/validation/tests/MapCSSTagChecker.java	(revision 7298)
@@ -503,5 +503,5 @@
      * @return all errors for the given primitive, with or without those of "info" severity
      */
-    public Collection<TestError> getErrorsForPrimitive(OsmPrimitive p, boolean includeOtherSeverity) {
+    public synchronized Collection<TestError> getErrorsForPrimitive(OsmPrimitive p, boolean includeOtherSeverity) {
         final ArrayList<TestError> r = new ArrayList<>();
         final Environment env = new Environment(p, new MultiCascade(), Environment.DEFAULT_LAYER, null);
@@ -585,5 +585,5 @@
 
     @Override
-    public int hashCode() {
+    public synchronized int hashCode() {
         final int prime = 31;
         int result = super.hashCode();
@@ -593,5 +593,5 @@
 
     @Override
-    public boolean equals(Object obj) {
+    public synchronized boolean equals(Object obj) {
         if (this == obj)
             return true;