rain on parade
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rain on (one's) parade
To ruin one's plans or temper one's excitement. I hate to rain on your parade, but I think your A in chemistry was actually a clerical error. Mom really rained on our parade by chaperoning our school dance.
rain on someone's paradeand rain on someone or something
Fig. to spoil something for someone. I hate to rain on your parade, but your plans are all wrong. She really rained on our plans.