If you manually kill an EC2 instance that hudson was using, it gets stuck.
Trying to delete the node in hudson results in:

ava.lang.NullPointerException
at hudson.plugins.ec2.EC2Computer.doDoDelete(EC2Computer.java:92)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.jav
a:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at
org.kohsuke.stapler.Function$InstanceFunction.invoke(Function.java:185)
at org.kohsuke.stapler.Function.bindAndInvoke(Function.java:101)
at
org.kohsuke.stapler.Function.bindAndInvokeAndServeResponse(Function.java:54)
at org.kohsuke.stapler.MetaClass$1.doDispatch(MetaClass.java:74)
at
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:487)
at org.kohsuke.stapler.MetaClass$8.dispatch(MetaClass.java:215)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:487)
at org.kohsuke.stapler.MetaClass$4.doDispatch(MetaClass.java:144)
at
org.kohsuke.stapler.NameBasedDispatcher.dispatch(NameBasedDispatcher.java:30)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:487)
at org.kohsuke.stapler.Stapler.invoke(Stapler.java:403)
at org.kohsuke.stapler.Stapler.service(Stapler.java:116)
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:94)
at hudson.util.PluginServletFilter.doFilter(PluginServletFilter.java:86)
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.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 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:595)