Create a saved search folder on your inbox (for example I have one for addresses not in certain address books)
Open this saved search
Hit delete on the first message
At times (and I'm not yet sure what the conditions are) the the list does not update.
Since messages are often in collapsed threads, its hard to tell visually if the top message is now the deleted message, or is another one in the same thread.
If you click down arrow to go to the next message, the UX updates, and you end up skipping a message -
To be clearer
List shows messages A B C
Click on A ; hit Delete
List shows A B C still
Click down arrow
List shows B and C and C is highlighted
It is not 100% repeatable, and I'm not sure quite what conditions cause it to happen, but its frequent enough that I've deleted the wrong message more than once (which is how I found out that Undo is missing)
Note - this is a potentially data-losing bug, so maybe worth more atention than it might otherwise suggest.