From 5229 of the uncompressed jQuery.. 

2. try {
3.         var oldAbort = xhr.abort;
4.         xhr.abort = function() {
5.           if (xhr) {
6.             oldAbort.call(xhr);
7.           }
8. 
   
9.           onreadystatechange("abort");
10.         };
11.       } catch (e) { 

Line # 6 above is what is throwing an error in IE's debugger. Yes, its in a try/catch, but that isn't stopping IE from throwing an error and preventing my $.ajax error function from being called. 

Edit: it appears that since its in a callback function the try/catch doesn't have an effect (apparently I'm still not awake this morning). 

My code looks like.. 

2. xhr = $.ajax({
3.           url: baseUrl + "/Gears/online",
4.           cache: false,
5.           dataType: 'text',
6.           timeout: 7000,
7.           error: function(xhr, errorDesc) {
8.             // Error
9.             location.href = baseUrl + "/Account/Logon";
10.           },
11.           success: function(data, status, xhr) {
12.             // Able to retrieve a request from the server
13.             //debugger;
14.             if (xhr.status == 403) {
15.               // not logged in, redirect to login page
16.               location.href = baseUrl + "/Account/Logon";
17.               return;
18.             }
19.             else if (data == "OK") {
20.               // System is online
21. 
    
22.               if (_backOnlineReAuth) {
23.                 // changed user, reauth
24.                 location.href = baseUrl + "/Account/LogOff";
25.                 return;
26.               }
27. 
    
28.               // Good to go!
29.               if (_isOnline) return;
30. 
    
31.               // Status change: Offline -> Online
32.               _goOnline();
33.               console.notice("Back Online", "Connection restored");
34.             }
35.           },
36.           complete: function() {
37.             // Continue with the next item in the app start-up queue
38.             nextQueue();
39.           }
40.         });
41.       }