I'm in the process of creating a somewhat complicated configuration screen to assign users to different lists, dynamically creating groups of users and saving this configuration.
For this purpose i'm using multiple sortables, linked by "connectWith". For groups i dynamically create sub lists (ul in a li) wich is also a sortable.
When using a nested sortable, it's possible to drop an item from a connected sortable in the nested sortable, without triggering it's "over" event. (i use this over event to check if the dragged element is allowed there. If not a cancel event is fired).
I've created a (stripped down) JSFiddle as example. There are 2 lists, one with a nested list. If you (gently) try to drag an element from the right list to the top of the left nested list (so as the first item) you can see in the console that sometimes either no "over' event is fired or the "over" event of the parent sortable is called.
If you release the dragged element, it will actually get added to the nested sortable, without ever firing the over event. Something that should not be possible in my opinion.
This example has no styling. If you were to add more styling (as i have in my project) this problem gets worse because of padding/marings etc.
The over event seems to fire only if "enough" of the dragged element is over the nested sortable.