I do: "find ~ -iname ":" | wc -l" yields 49 entries. Some of those are Xorg-related (which identifies displays with a syntax like ":0.0", which ends up in some log file names) or gvfs stuff. But most are PDFs, usually research papers which I tend to save as "title: subtitle - authors.pdf".
High score! In all my filesystem I get about 150,000 files with a colon in their name. Many of them are part of the filesystems of containers which seem to use colons in the names of their libraries and dpkg packages. Many archlinux packages have colons in their names too. I also have some media files and documents with colons in names too.
It's actually really common, for someone who's used *nix for ages and expects every character to be a valid filename.
