Thanks RamPackWobble! It was effects. I don't really need effects to try things out - so I removed them, saved my project, closed Construct 2, then opened it again and was back in business!
I hear you Dutoit on purchasing a license - and I'm almost ready to do this. What's holding me back are the many bugs I've found in the product.
I've encountered a bug that prevents the events sheet from being re-interpreted when running again (boy that was annoying), a bug that stops my game from running at all, and a bug that hangs C2 during an Export - all in the latest build 163. All of these the bugs were cleared up by closing and re-starting C2 - but as the saying goes, "where there's smoke, there's fire!" I'm worried about investing a ton of time in a game on C2 then having a serious problem crop up and impact my work. $100 is a lot of money for a product with as many issues as I've found.
I've also seen several one-time bugs (i.e. I couldn't reproduce them) that included a message to please "send this bug to the developer" - and I was a little surprised they didn't offer me a button to do exactly that. Clearly C2 knew something was wrong - why not instrument the product to collect these incidents?
That said, C2 seems to be much easier to use than Game Maker Studio - and I haven't encountered any issues that couldn't be fixed by restarting - so Scirra may get my dollars yet.