Denarius of Julia Maesa has an ill-defined empress-shaped incuse area behind a well-struck Juno on the reverse. Any idea what happened? A brockage that was restruck? Die clash? Something else? The die axis is about 15 degrees and it closely matches the bust on the obverse but not exactly.
My favorite errors of any sort, die clashes. Thats a cool one too! They are a bit common on ancients. I have a few.
100% die clash. It seems that many dies of this period were clashed. The big periods for clashes are scattered around suggesting to me that some mint officials destroyed damaged dies and some could care less. After the Severans, we see fewer clashes until they peak again under Claudius II (perhaps even more than late Gallienus). You will have a hard time gathering a set of emperors with clashes. This is my Maesa clash. I assume you have seen my page: http://www.forumancientcoins.com/dougsmith/brock.html
Most informative, as always, @dougsmit. Thank you so much! What an educational and relevant page you have on the subject of die clashes!