I never knew I could write a novel in 30 days - until National Novel Writing Month came along and suggested I give it a try. I liked it so much, I've done it 3 times now.
In a similar vein, some colleagues and I were recently invited to create a single-panel cartoon (ala Far Side) for everyone's favorite technology journal, Defense AT&L. We didn't know we could do it until we were asked - and then we came up with over 30 comic ideas in about 2 weeks (they only publish 6 issues a year, so that's like a 5 year supply... except some of the ideas probably aren't funny, so maybe it's just a 2 year supply).
I wonder what else I'm not doing, not creating, simply because I didn't think of it.