This has just started happening to me over the last month or so, unrelated to mood or fears. Sims with level 10 cooking skill, flash fully upgraded ovens, no fear of fire, confident and inspired are catching fire at about a rate of 50/50 when using their ovens or grills. Happening in all households I've played with across current game. It seems that to cook, they have to not be hungry (Sim this just happened to had high moodlets excepting a yellow bar for hunger, Level 10 cooking skill and Level 8 gourmet cooking and using a Schmapple Oven). Both Sim and oven or grill are catching fire. I'm genuinely irritated by this and I'm getting tense every time I send a Sim to cook, which is not a fun way to play. It's not interesting to make everyone prepare salads instead of hot foods, especially during a blizzard!
The Sims Team
