Nutrition/Calories

If it's a rare day (once a week, say), and only a little under (50 calories, say), it's OK, as long as you're not losing weight too fast (max of 1% of current body weight weekly for most people, but suggested max may be lower if you have less than 50 or so pounds to lose, total). That's especially true if on some other day, you're a little over, so that things balance out over the week.

If it's frequent, then eat something calorie dense but not too filling, such as nuts, avocado, peanut butter, a little extra oil on salad or veggies, etc. Some people find drinking calories less filling than eating them (can be nutritious things, if you prefer, like juice, kefir, smoothies, etc.).

If your nutrition is on point, and you have calories left, it's also fine to eat a less nutrient-dense treat food (cookie or candy or ice cream, for example), assuming that those are not trigger foods you have difficulty moderating.

Regularly, persistently undereating increases various kinds of health risks. No one wants that!