Are you using the "Dodge & Burn" plug-in? What is your Aperture version? Some plug-ins are no longer compatible with Aperture 3.4: Aperture 3.4 and later: Some third-party plug-ins are no longer compatible
I.e. the Dodge&Burn plug-in has not been working properly with Aperture since the release of Lion. Re: Aperture unexpectedly quits when I select saturation within "dodge ¶ burn"...any idea?
But if you are using Aperture 3 and not Aperture 2, you can use the built-in "Dodge" and "Burn" adjustments in Aperture 3.
If it are the built in brushes that are crashing - have you upgraded to the latest Aperture version? You may have missed some bug fixes.