Reentrant:

void A() {
...
DoEvents(); // -> this might call A()
...
}

Trouble:

int i;
void B(array<T> j) {
   for( i = 0; i < j.Length; i++ ) {
       DoEvents();    // this might change i
       j[i].ToString();    // which could cause an array index bounds
exception here
   }
}

You have to understand what your application is doing. :-)

Seriously, say for example, that you have an application with a menu which
has an item that causes a function to run. Say that it is in that function
where you intend to put the little "wait" snippet that I sketched. If you
did nothing else but insert that snippet, it would be possible for the user
to pull down the menu and click the menu item which would cause the function
to be called a second time before the first call returns.

That would be bad but it is not the end of the world. The simple fix in a
case like that would be to disable the menu item, run the function and then
reenable it.

It's not at all unusual in places where you find such secondary message
loops to find some disabling of UI widgets before the loop runs followed by
reenabling afterwards. Exactly what kind of fiddling you need to do depends
on exactly what your application does.

Regards,
Will