Steps to reproduce:

1. Configure an existing project and add a build step to copy artifacts from another project
2. Enter the project name to copy artifacts from
*3. Leaving all other fields blank enter a path as the value in the "Target directory" field e.g. "cb20_cfDesignServlet\WEB-INF\lib" without the quotes
4. Now add an additional copy artifacts build step and enter another project to copy artifacts from.
*5. Enter the same value as previously for Target directory i.e. "cb20_cfDesignServlet\WEB-INF\lib"
6. Click Save

*It seems immaterial whether a Target directory is specified or not.

The following exception is encountered:

net.sf.json.JSONException: A JSONObject text must begin with '{' at character 1 of init
net.sf.json.util.JSONTokener.syntaxError(JSONTokener.java:512)
net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:839)
net.sf.json.JSONObject._fromString(JSONObject.java:1060)
net.sf.json.JSONObject.fromObject(JSONObject.java:176)
net.sf.json.JSONObject.fromObject(JSONObject.java:147)
org.kohsuke.stapler.RequestImpl.getSubmittedForm(RequestImpl.java:711)
hudson.model.Job.doConfigSubmit(Job.java:1026)
hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:551)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
java.lang.reflect.Method.invoke(Unknown Source)
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:169)
org.kohsuke.stapler.Function.bindAndInvoke(Function.java:101)
org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:54)
org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:74)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:519)
org.kohsuke.stapler.MetaClass$6.doDispatch(MetaClass.java:180)
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:519)
org.kohsuke.stapler.Stapler.invoke(Stapler.java:435)
org.kohsuke.stapler.Stapler.service(Stapler.java:123)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
hudson.util.PluginServletFilter$1.doFilter(PluginServletFilter.java:94)
hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:78)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)

Workaround:

Add copy-artifact build steps one at a time.