Others have mentioned gauge blocks that are wrung together.
I once read an article (sorry, no cite) that said physicists still do not understand the physical mechanism for why gauge blocks stick together.
The article said it was not due to Van Der Waal Forces. It also said it was not due to air pressure, since experiments have shown gauge blocks can be wrung together in a vacuum.
According to the article, the only explanation to-date is “molecular forces.”