What is a supposedly “once in a lifetime experience” that left you underwhelmed?

Went to the Louvre a few years ago and I was transfixed by the sheer volume of beautiful artwork all around me. Then we entered the Mona Lisa room and…..it was terrible. Not the painting, just everything. There were hundreds of people crammed into the space, and almost all of them were elbowing each other to get up front, snap a picture, then leave. You couldn’t even enjoy it. The rest of the museum was wonderful though.