diff --git a/common/content/options.js b/common/content/options.js
index 34ce63bf..58e529b1 100644
--- a/common/content/options.js
+++ b/common/content/options.js
@@ -8,11 +8,7 @@
 
 /** @scope modules */
 
-let ValueError = Class("ValueError", Error, {
-    init: function (message) {
-        update(this, Error(message));
-    }
-});
+let ValueError = Class("ValueError", ErrorBase);
 
 // do NOT create instances of this class yourself, use the helper method
 // options.add() instead
diff --git a/common/modules/base.jsm b/common/modules/base.jsm
index dfcf7c94..7d393be2 100644
--- a/common/modules/base.jsm
+++ b/common/modules/base.jsm
@@ -176,7 +176,7 @@ function require(obj, name, from) {
 defineModule("base", {
     // sed -n 's/^(const|function) ([a-zA-Z0-9_]+).*/	"\2",/p' base.jsm | sort | fmt
     exports: [
-        "Cc", "Ci", "Class", "Cr", "Cu", "Module", "Object", "Runnable",
+        "ErrorBase", "Cc", "Ci", "Class", "Cr", "Cu", "Module", "Object", "Runnable",
         "Struct", "StructBase", "Timer", "UTF8", "XPCOM", "XPCOMUtils", "array",
         "call", "callable", "ctypes", "curry", "debuggerProperties", "defineModule",
         "deprecated", "endModule", "forEach", "isArray", "isGenerator",
@@ -857,6 +857,26 @@ function XPCOM(interfaces, superClass) {
     return res;
 }
 
+/**
+ * An abstract base class for classes that wish to inherit from Error.
+ */
+const ErrorBase = Class("ErrorBase", Error, {
+    level: 2,
+    init: function (message, level) {
+        level = level || 0;
+        update(this, Error(message))
+        this.message = message;
+
+        let frame = Components.stack;
+        for (let i = 0; i < this.level + level; i++) {
+            frame = frame.caller;
+            this.stack = this.stack.replace(/^.*\n/, "");
+        }
+        this.fileName = frame.filename;
+        this.lineNumber = frame.lineNumber;
+    }
+});
+
 /**
  * Constructs a new Module class and instantiates an instance into the current
  * module global object.
diff --git a/common/modules/styles.jsm b/common/modules/styles.jsm
index c23a7c65..1049b899 100644
--- a/common/modules/styles.jsm
+++ b/common/modules/styles.jsm
@@ -356,17 +356,39 @@ const Styles = Module("Styles", {
 }, {
     commands: function (dactyl, modules, window) {
         const commands = modules.commands;
+
+        const queue = [];
+        const timer = Timer(10, 10, function () {
+            let args = queue.shift()
+            let [filter, css] = args;
+            if ("-append" in args) {
+                let sheet = styles.user.get(args["-name"]);
+                if (sheet) {
+                    filter = sheet.sites.concat(filter).join(",");
+                    css = sheet.css + " " + css;
+
+                }
+            }
+            styles.user.add(args["-name"], filter, css, args["-agent"]);
+
+            if (queue.length)
+                timer.tell();
+        });
         commands.add(["sty[le]"],
             "Add or list user styles",
             function (args) {
                 let [filter, css] = args;
-                let name = args["-name"];
 
-                if (!css) {
+                if (css) {
+                    queue.push(args);
+                    timer.tell(args);
+                }
+                else {
                     let list = styles.user.sheets.slice()
                                      .sort(function (a, b) a.name && b.name ? String.localeCompare(a.name, b.name)
                                                                             : !!b.name - !!a.name || a.id - b.id);
                     let uris = util.visibleURIs(window.content);
+                    let name = args["-name"];
                     modules.commandline.commandOutput(
                         template.tabular(["", "Name", "Filter", "CSS"],
                             ["min-width: 1em; text-align: center; color: red; font-weight: bold;",
@@ -379,17 +401,6 @@ const Styles = Module("Styles", {
                              for (sheet in values(list))
                              if ((!filter || sheet.sites.indexOf(filter) >= 0) && (!name || sheet.name == name)))));
                 }
-                else {
-                    if ("-append" in args) {
-                        let sheet = styles.user.get(name);
-                        if (sheet) {
-                            filter = sheet.sites.concat(filter).join(",");
-                            css = sheet.css + " " + css;
-
-                        }
-                    }
-                    styles.user.add(name, filter, css, args["-agent"]);
-                }
             },
             {
                 bang: true,
diff --git a/common/modules/util.jsm b/common/modules/util.jsm
index 8a5c6471..88f2de29 100644
--- a/common/modules/util.jsm
+++ b/common/modules/util.jsm
@@ -28,12 +28,7 @@ memoize(this, "Commands", function () {
     return obj.Commands;
 });
 
-const FailedAssertion = Class("FailedAssertion", Error, {
-    init: function (message) {
-        update(this, Error(message))
-        this.message = message;
-    }
-});
+const FailedAssertion = Class("FailedAssertion", ErrorBase);
 
 function wrapCallback(fn)
     fn.wrapper = function wrappedCallback () {
@@ -126,7 +121,7 @@ const Util = Module("Util", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference])
      */
     assert: function (condition, message) {
         if (!condition)
-            throw FailedAssertion(message);
+            throw FailedAssertion(message, 1);
     },
 
     get chromePackages() {