Recently found one thing. Perhaps in docs for AJAX client or somewhere else was such line:

Sys.Application.notifyScriptLoaded(); 

You need to write this line at the end of your JS files. But this can cause the problems if you wish to re-use this code in non-ajax enviroment. For this better to use following line:

if (typeof(Sys) !== 'undefined')  Sys.Application.notifyScriptLoaded(); 

This makes things a bit better. Also, it answers another question - "Testing for defined variables in Javascript".  Sometimes I see the code like:

if( Sys )...  // And this will fail in case Sys is undefined. 

 That's all for now, it's summer. Everybody enjoy their resort life. But I'm still working.