The help on the "# of executors field" states:

Setting this value to 0 is useful to remove a disabled slave from Hudson
temporarily without losing other configuration information.

When you set the value to 0, and go to another field, you get a validation warning:

Not a positive number

And if you save, you get the following error message:

Status Code: 500
Exception:
Stacktrace:

java.lang.IllegalArgumentException: hudson.model.Descriptor$FormException:
Invalid slave configuration for machinename – Windows XP Slave. Invalid # of
executors.
at org.kohsuke.stapler.RequestImpl.invokeConstructor(RequestImpl.java:392)
at org.kohsuke.stapler.RequestImpl.bindJSON(RequestImpl.java:323)
at org.kohsuke.stapler.RequestImpl.bindJSONToList(RequestImpl.java:337)
at hudson.model.Hudson.doConfigExecutorsSubmit(Hudson.java:1575)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:103)
at org.kohsuke.stapler.Function.bindAndinvoke(Function.java:57)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:75)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:440)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:361)
at org.kohsuke.stapler.Stapler.service(Stapler.java:121)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:45)
at winstone.ServletConfiguration.execute(ServletConfiguration.java:249)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:335)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:378)
at hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:38)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:30)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:52)
at
hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:28)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at
org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at
org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at
org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at
org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at
org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at
org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at
hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:42)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:55)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:44)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:85)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at winstone.RequestDispatcher.forward(RequestDispatcher.java:333)
at winstone.RequestHandlerThread.processRequest(RequestHandlerThread.java:244)
at winstone.RequestHandlerThread.run(RequestHandlerThread.java:150)
at java.lang.Thread.run(Thread.java:619)
Caused by: hudson.model.Descriptor$FormException: Invalid slave configuration
for machinename – Windows XP Slave. Invalid # of executors.
at hudson.model.Slave.(Slave.java:115)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at
sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at
sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at org.kohsuke.stapler.RequestImpl.invokeConstructor(RequestImpl.java:377)
... 46 more