Opponent attaching an energy from the discard pile can cause visual bug + short freeze
If you're looking at your opponent discard pile while they are using a trainer or ability that attaches energy from their discard pile (i have seen it work with Astral radiance Regigigas, Dark patch and Raihan) sometimes the discard pile screen closes after the opponent chooses an energy, the selected energy (or one of them in Regigigas case) starts glowing and freezes the game for around 5 seconds, after that the game proceeds as normal and the visual bug lasts until you click on any card.
The glow is as bad as the random flash the game used to have after taking prizes, is both annoying and potentially harmful for the player eyes.
As for why it happens, my guess is that trying to play the "leave the discard pile" and the "attach an energy" animation at the same time may be confusing the game.