Today, I added my first "tick-mark" feature to Mono's C# compiler: qualified-alias-member or '::' lookup. It's a relatively small feature, and doesn't impact much, code-wise.
In all, it took about a day's worth of effort from start to check-in. But, that one-day effort needed quite some preparatory work. I'd pretty much decided on the approach a couple of months ago, and had a reasonable plan-of-attack, and was looking for an opportune time to work on it. More importantly, the past few months I spent incessantly refactoring the name- and type-lookup facilities in the compiler has paid off, making the task of implementing '::' trivial.