Everything moderation.
For example, if, as a person who has been on a low carb diet for a very long time, I "moderate" my diet by adding a few small sandwiches each week, the sugar in the bread will cause my weight to shoot up (initially water gain), cause my blood pressure to rise, and even cause my bad cholesterol numbers to climb.
As my doctor put it, a little bit of sugar to a person on a keto diet is like a little bit of alcohol to an alcoholic (sugar is that addictive).