i think it will probably appear after every reassign. but in case that works, try reproducing with the following scenario:
1) start a new process instance
2) a start task instance was created for the start task
3) entered a couple of values
4) did Save and Close
5) then I was navigated to the next task
6) in that next task i did a reassign
then i got the following exception

org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: org.jbpm.taskmgmt.exe.TaskInstance.variableInstances, no session or session was closed
at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationException(AbstractPersistentCollection.java:358)
at org.hibernate.collection.AbstractPersistentCollection.throwLazyInitializationExceptionIfNotConnected(AbstractPersistentCollection.java:350)
at org.hibernate.collection.AbstractPersistentCollection.readIndexExistence(AbstractPersistentCollection.java:115)
at org.hibernate.collection.PersistentMap.containsKey(PersistentMap.java:109)
at org.jbpm.context.exe.VariableContainer.hasVariableLocally(VariableContainer.java:119)
at org.jbpm.context.exe.VariableContainer.getVariable(VariableContainer.java:28)
at org.jbpm.context.exe.VariableContainer$$FastClassByCGLIB$$56c4a5bb.invoke(<generated>)
at net.sf.cglib.proxy.MethodProxy.invoke(MethodProxy.java:149)
at org.hibernate.proxy.pojo.cglib.CGLIBLazyInitializer.intercept(CGLIBLazyInitializer.java:163)
at org.jbpm.taskmgmt.exe.TaskInstance$$EnhancerByCGLIB$$32f29d9a.getVariable(<generated>)
at org.jbpm.webapp.bean.TaskInstanceVariableMap.get(TaskInstanceVariableMap.java:37)
at javax.el.MapELResolver.getValue(MapELResolver.java:157)
at javax.el.CompositeELResolver.getValue(CompositeELResolver.java:135)
at com.sun.faces.el.FacesCompositeELResolver.getValue(FacesCompositeELResolver.java:64)
at com.sun.el.parser.AstValue.getValue(AstValue.java:96)
at com.sun.el.ValueExpressionImpl.getValue(ValueExpressionImpl.java:183)
at com.sun.facelets.el.TagValueExpression.getValue(TagValueExpression.java:71)
at javax.faces.component.UIOutput.getValue(UIOutput.java:173)
at com.sun.faces.renderkit.html_basic.HtmlBasicInputRenderer.getValue(HtmlBasicInputRenderer.java:189)
at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.getCurrentValue(HtmlBasicRenderer.java:320)
at com.sun.faces.renderkit.html_basic.HtmlBasicRenderer.encodeEnd(HtmlBasicRenderer.java:200)
at javax.faces.component.UIComponentBase.encodeEnd(UIComponentBase.java:829)
at org.jbpm.webapp.tag.jbpm.ui.ComponentBase.doEncode(ComponentBase.java:48)
at org.jbpm.webapp.tag.jbpm.ui.ComponentBase.doEncode(ComponentBase.java:45)
at org.jbpm.webapp.tag.jbpm.ui.DataForm.renderCells(DataForm.java:117)
at org.jbpm.webapp.tag.jbpm.ui.DataForm.encodeChildren(DataForm.java:95)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:884)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:890)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:890)
at javax.faces.render.Renderer.encodeChildren(Renderer.java:137)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:810)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:884)
at javax.faces.render.Renderer.encodeChildren(Renderer.java:137)
at javax.faces.component.UIComponentBase.encodeChildren(UIComponentBase.java:810)
at org.jbpm.webapp.tag.jbpm.ui.ComponentBase.doEncode(ComponentBase.java:33)
at org.jbpm.webapp.tag.jbpm.ui.ComponentBase.doEncode(ComponentBase.java:58)
at org.jbpm.webapp.tag.jbpm.ui.Tab.encodeChildren(Tab.java:112)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:884)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:890)
at javax.faces.component.UIComponent.encodeAll(UIComponent.java:890)
at com.sun.facelets.FaceletViewHandler.renderView(FaceletViewHandler.java:578)
at org.jbpm.webapp.application.JbpmViewHandler.renderView(JbpmViewHandler.java:58)
at com.sun.faces.lifecycle.RenderResponsePhase.execute(RenderResponsePhase.java:106)
at com.sun.faces.lifecycle.LifecycleImpl.phase(LifecycleImpl.java:248)
at com.sun.faces.lifecycle.LifecycleImpl.render(LifecycleImpl.java:144)
at javax.faces.webapp.FacesServlet.service(FacesServlet.java:245)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.jbpm.webapp.filter.LogFilter.doFilter(LogFilter.java:59)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:202)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.jboss.web.tomcat.security.SecurityAssociationValve.invoke(SecurityAssociationValve.java:175)
at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:524)
at org.jboss.web.tomcat.security.JaccContextValve.invoke(JaccContextValve.java:74)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:869)
at org.apache.coyote.http11.Http11BaseProtocol$Http11ConnectionHandler.processConnection(Http11BaseProtocol.java:664)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.MasterSlaveWorkerThread.run(MasterSlaveWorkerThread.java:112)
at java.lang.Thread.run(Thread.java:595)