I was playing a game of cube.

My opponent cast a Gonti, exiling and then later playing the Serra Paragon from my deck.

They had a Prairie Whatsit in play (the 0/1 flyer that permanently adds +1/0 to a creature every turn) and at some point gave the Serra Paragon +1/0.

I then used a channeled Touch the Spirit Realm to exile the Serra Paragon and 'return it to its owner's control at the beginning of the next end step'.

When the Serra Paragon re-entered play it was still under my opponent's control.

I lost the game. These games cost money.

P.S. The process for submitting this bug report was appallingly unfriendly, and your company and its decision makers should be ashamed of the terrible usability of the process. Cheers.