I was pretty new before, but I am starting to get the hang of this.
A few tips
1) Try not to add certain complex objects unless you have to, hence use duplicate instead when possible, because a misclick can cause a malformed XML tag to be inserted.
2) If you think the SDK inserted a malformed XML tag, it's best to open the XML yourself and look for something like </object> (apron, whatever it is), and then see if the beginning tag is also missing. Then remove the extra empty tag or malformed tags before restarting the editor.
3) There is a "click bug" to where when you have one of the windows open over the scenery, sometimes clicking somewhere in the window is received by the window behind it which changes the actively selected object. This does not seem to always happen, and I'm not sure what causes it. It seems to happen more with really large objects, like giant aprons you make for texturing.
4) There is a bug or "feature" to where the windows and selections are locked by category. Hence, sometimes you have to select the object category from the objects list (like select airport). i don't know if I am hitting a key that is changing selection mode or what I am doing that is causing that, but I just work around it.
5) There is a CTD bug that occurs sometimes when you are trying to reposition windows and it appears the application thinks you are clicking on two windows at once (it seems to be loosely related to the issue in # 4). Though it may also happen from a malformed XML (do not recall).
Hopefully that might help someone else that is just starting like I was before.
My advice is save often and keep backups of the files.