This first example, with three colons, works fine:
<small>This text is small :::</small>This text is normal The rest of the page is fine.
This second example, with four colons, breaks the entire page:
<small>This text is small ::::</small>This text is small, but it shouldn't be. This text, and the entire page, is incorrectly rendered as small.
I ran into this issue when viewing EnWiki's WP:Administrators's_noticeboard/Archive267. This bizarre result appears to be relatively recent breakage in the parser.