When the html5-notifier plugin is enabled, you cannot save a job with more than one build step. To reproduce, install the plugin and add a new job with two windows batch file or shell script build steps. If only one is created, it will save, but if two are created, you will see an error like the following:

Failed to parse form data. Please report this problem as a bug
JSON={"":"","builder":"\"[

{\"command\":\"bar\",\"stapler-class\":\"hudson.tasks.Shell\",\"kind\":\"hudson.tasks.Shell\"}

,

{\"command\":\"cd foo\",\"stapler-class\":\"hudson.tasks.Shell\",\"kind\":\"hudson.tasks.Shell\"}

]\"","description":"","hasSlaveAffinity":

{"assignedLabelString":"windows&&expander"}

,"name":"debug_job2","properties":{"hudson-model-ParametersDefinitionProperty":{},"hudson-plugins-batch_task-BatchTaskProperty":{},"stapler-class-bag":"true"},"scm":{"value":"3"}}

net.sf.json.JSONException: A JSONArray text must start with '[' at character 1 of "[

{"command":"cd bar","stapler-class":"hudson.tasks.Shell","kind":"hudson.tasks.Shell"}

,

{"command":"cd foo","stapler-class":"hudson.tasks.Shell","kind":"hudson.tasks.Shell"}

]"
at net.sf.json.util.JSONTokener.syntaxError(JSONTokener.java:512)
at net.sf.json.JSONArray._fromJSONTokener(JSONArray.java:903)
at net.sf.json.JSONArray._fromString(JSONArray.java:983)
at net.sf.json.JSONArray.fromObject(JSONArray.java:141)
at net.sf.json.JSONArray.fromObject(JSONArray.java:120)
at hudson.model.Descriptor.newInstancesFromHeteroList(Descriptor.java:863)
at hudson.model.Descriptor.newInstancesFromHeteroList(Descriptor.java:853)
at hudson.util.DescribableList.rebuildHetero(DescribableList.java:185)
at hudson.model.Project.submit(Project.java:197)
at hudson.model.Job.doConfigSubmit(Job.java:966)
at hudson.model.AbstractProject.doConfigSubmit(AbstractProject.java:643)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:616)
at org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:282)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:149)
at org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:88)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:104)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:561)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
at org.kohsuke.stapler.MetaClass$6.doDispatch(MetaClass.java:234)
at org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:53)
at org.kohsuke.stapler.Stapler.tryInvoke(Stapler.java:561)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:646)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:477)
at org.kohsuke.stapler.Stapler.service(Stapler.java:159)
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:95)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:87)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.csrf.CrumbFilter.doFilter(CrumbFilter.java:47)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:84)
at hudson.security.UnwrapSecurityExceptionFilter.doFilter(UnwrapSecurityExceptionFilter.java:51)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.ExceptionTranslationFilter.doFilter(ExceptionTranslationFilter.java:166)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.providers.anonymous.AnonymousProcessingFilter.doFilter(AnonymousProcessingFilter.java:125)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.rememberme.RememberMeProcessingFilter.doFilter(RememberMeProcessingFilter.java:142)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.AbstractProcessingFilter.doFilter(AbstractProcessingFilter.java:271)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.ui.basicauth.BasicProcessingFilter.doFilter(BasicProcessingFilter.java:173)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at jenkins.security.ApiTokenFilter.doFilter(ApiTokenFilter.java:61)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at org.acegisecurity.context.HttpSessionContextIntegrationFilter.doFilter(HttpSessionContextIntegrationFilter.java:249)
at hudson.security.HttpSessionContextIntegrationFilter2.doFilter(HttpSessionContextIntegrationFilter2.java:66)
at hudson.security.ChainedServletFilter$1.doFilter(ChainedServletFilter.java:87)
at hudson.security.ChainedServletFilter.doFilter(ChainedServletFilter.java:76)
at hudson.security.HudsonFilter.doFilter(HudsonFilter.java:164)
at winstone.FilterConfiguration.execute(FilterConfiguration.java:195)
at winstone.RequestDispatcher.doFilter(RequestDispatcher.java:368)
at hudson.util.CharacterEncodingFilter.doFilter(CharacterEncodingFilter.java:81)
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:636)

I think the problem is the \" at the start that's inserted there...

Workaround is to uninstall the plugin.