[ctrlrsf]

How do you even know this?

[ska]

Spend some time doing math and you'll pick up a few without noticing. Somewhat path dependent, but they do crop up.

See also: https://en.wikipedia.org/wiki/List_of_mathematical_constants

[77pt77]

Some of those are quite obscure

[ska]

And some aren't; it's not a list of "commonly encountered" constants, just a list.