I've heard that brushing right after eating is really bad for your teeth because just after you eat, the bacteria in your mouth start producing acid that, for lack of a better word, loosens the surface enamel in your teeth, and brushing in that state discards that enamel instead of letting it resettle after the acid washes away.

The solubility of enamel is insanely low so the effect should be minor at most, but still.