All custom build target rebuilt after a single custom target fails in VS2010 - workaround?

We've run into a problem with VS2010SP1 where if a custom build target fails, then on the next build, all custom targets get rebuilt. This really slows down development. 

It sure looks like the custombuild rule is mishandling custombuild.command.1.tlog. Any suggestions? (One custom build target per project won't work)

I am able to repro the problem in a small test app. Our sample custom build rule looks like:

  <ItemGroup>
    <CustomBuild Include="cow1.moo">
      <FileType>Document</FileType>
      <Command>"./Bovinator.py" -o "./$(ConfigurationName)/%(Filename).txt" "%(FullPath)"</Command>
      <Message>Bovinating</Message>
      <Outputs>./$(ConfigurationName)/%(Filename).txt</Outputs>
    </CustomBuild>
    <CustomBuild Include="cow2.moo">
      <FileType>Document</FileType>
      <Command>
        "./Bovinator.py" -o "./$(ConfigurationName)/%(Filename).txt" "%(FullPath)"</Command>
        <Message>Bovinating</Message>
        <Outputs>./$(ConfigurationName)/%(Filename).txt</Outputs>
      </CustomBuild>
  </ItemGroup>