with [selectNodes -cache 1], when a syntactically-invalid XPath expression is used twice, no error is raised the second time, and an empty expression is used by xPathEval. Here is a patch:

Index: generic/domxpath.c
==================================================================
--- generic/domxpath.c
+++ generic/domxpath.c
@@ -5187,20 +5187,25 @@
 )
 {
     xpathResultSet nodeList;
     int            rc, hnew = 1, docOrder = 1;
     ast            t;
-    Tcl_HashEntry *h;
+    Tcl_HashEntry *h = NULL;
 
     *errMsg = NULL;
     if (cache) {
         h = Tcl_CreateHashEntry (cache, xpath, &hnew);
     }
     if (hnew) {
         rc = xpathParse(xpath, exprContext, XPATH_EXPR, prefixMappings,
                         parseVarCB, &t, errMsg);
-        CHECK_RC;
+        if (rc) {
+            if (h != NULL) {
+                Tcl_DeleteHashEntry(h);
+            }
+            return rc;
+        }
         if (cache) {
             Tcl_SetHashValue(h, t);
         }
     } else {
         t = (ast)Tcl_GetHashValue(h);

You're right. Fix applied. Thanks.