[jms18]

What is wrong with spaces in URLs?

[eCa]

You need an (implicit or explicit) url encoding for it to work. The actual link to the linked article is[1] which is quite ugly, at least imho.

Much easier for everyone involved to just stick a dash in there.

[1] http://www.gwern.net/Archiving%20URLs

[thorum]

I think somewhere along the way my brain started processing %20 as a space, because I don't even notice anymore.

[sedev]

It might also be that browsers have started displaying it as a space - my Firefox does, for example (until you select it - copy-pasting it preserves the %20).