[unkown-unknowns]

I come across these once a year or so on the net but when I need them I never can remember them because it's not so often that I need them

[vlunkr]

Fortunately you only need to remember ~? since it tells you the rest.

[Groxx]

Unfortunately, ~? is two characters too many for me to remember a year later :|

[gumby]

If you need them but don’t remember them just use man ssh in another terminal.