In which I serve as a warning to others

1. When assigning a value to the innerHTML property of any element that allows it on IE, if the string contains a <form> element, Microsoft IE6 throws an alert with "unknown runtime error".
2. On Firefox, if you use a popup that makes callback to the window.opener and instantiates an XMLHttpRequest from within that callback, subsequent invocations of XMLHttpRequest fail with an internal exception. This has been a known issue since July 2004. The workaround for this is to have your callback function do setTimeout(0, "actualCallbackCode(/*args...*/)"), which (as I understand it) executes the callback within the original scriptContext instead of that of the popup's.
   
3. Similarly, calls to window.open after having callback code executed in FF can fail with the same error message when the url is relative instead of absolute. The workaround for this is to compute an absolute URL from document.location.
4. The IE team (much as I appreciate the hard work they do, etc.</diplomacy>) should scrap their scripting engine completely and use seamonkey's. The IE engine is old, crufty, nasty to debug in, and even when you have the (ancient) debugger installed(which also installs a different version of the scripting engine than that used by non-debugger-using users version of IE, making some bugs stupidly, braindeadeningly hard to pin down) tells you the line number of the error but not the file it occured in,and even gives you the relative line number of the error in the case of inline script, which is stupid.
   

Lessons learnt:

• I spent more time hunting for the FF bug (which was documented) than for the IE bug (which isn't). Yet because FF is OS, and IE is closed, IE is more annoying.
  
• innerHTML is like the easy path that leads to ruin, whereas true AJAX (i.e. with the response actually being XML and not JS or HTML fragments) is probably nicer and easier to work with yet harder to write in the first place.
• prototype.js is a wonderful library but only deals with responseText, never responseXML. This is something I would like to fix.