Hmmm you mean like an accidental thing? I was THINKING of something purposefully bad for the drama (since Gale is... slightly more familiar with that than he should be, given his route) but if Astarion has enough drama in his life with his divorce I could lean Gale losing control of a spell. Out of control fire or a storm that then catches things on fire or... a straight up explosion that Gale shields them from but misjudges how big it is or something!