a funny - When you first think something is funny, but then it hits yo

When you first think something is funny, but then it hits you like a brick and you realize it's actually a disaster.

I thought my idea was hilarious, but then I realized it would cost me $1000.

I thought my joke was funny, but my friend just stared at me like I was crazy.

I thought my plan was perfect, but it turned into a complete mess.