Doppleganger
I played Doppleganger for x = 3, and targeted 3 permanents on the field. One of them being my own legendary creature (which was enchanted), one was the enchantment (a pacifism like enchantment), and the final permanent was an opponent's creature. The next step was to attach the 3 enchantments to other permanents on the field, at which I became stuck in the selection screen. I choose 3 valid options, but it would not continue resolving Doppleganger. So naturally I attempted to select the 4th glowing card thinking I made a mistake. After this point, everything froze and I wound up using my time outs. This was frustrating for myself and my opponent as well. Eventually it registered as some auto-selection and bound 2 more copies of the pacifism like enchantment to my legendary creature, and did not copy anything else selected (e.g. my own legendary creature which upon entering, should have destroyed my pacified legendary creature, or even my opponent's creature).