What are the effects of one bad meal?
As usual, it depends.
How much work have you put in to your diet, for how long?
How well do you recover, physically and mentally?
What is one bad meal for you?
Like most things in life, it’s a spectrum. Here are two hypothetical examples:
A competitive bodybuilder who eats well year round can bounce back pretty quickly from a handful of french fries. That’s not likely to ruin all of his dietary efforts.
A person who just started removing processed foods from their diet a week ago, and then makes a binge trip to the fast food drive through is at much greater risk to derail their progress, simply because the habits & discipline aren’t yet strong enough.
Most of the time, that thing you’re eyeballing isn’t worth it anyway. But it’s not realistically possible, or advisable, to go through life with zero moments of indulgence with food.
When you’re considering your one bad meal, knowing where you are on the scale of risk is a good way to frame it.