Not sure if this should be handled in openwfe or if it should be handled in magnolia.
If a filter sets a request attribute with value "true" (as a string, not a boolean) workflow doesn't work anymore: the following stacktrace is what happens when doing an activation:
(spring filters always set this attribute...)

ERROR info.magnolia.module.workflow.WorkflowUtil WorkflowUtil.java(proceed:172) 20.02.2007 14:20:51 Error while accessing the workflow engine
java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Boolean
at openwfe.org.engine.workitem.BooleanAttribute.booleanValue(BooleanAttribute.java:94)
at openwfe.org.engine.workitem.BooleanAttribute.clone(BooleanAttribute.java:99)
at openwfe.org.engine.workitem.MapAttribute.clone(MapAttribute.java:215)
at openwfe.org.engine.workitem.MapAttribute.clone(MapAttribute.java:206)
at openwfe.org.engine.workitem.InFlowWorkItem.clone(InFlowWorkItem.java:224)
at openwfe.org.engine.expressions.SubProcessRefExpression.doLaunch(SubProcessRefExpression.java:627)
at openwfe.org.engine.expressions.SubProcessRefExpression.innerLaunch(SubProcessRefExpression.java:549)
at openwfe.org.engine.expressions.SubProcessRefExpression.apply(SubProcessRefExpression.java:268)
at openwfe.org.engine.expressions.raw.RawExpression.apply(RawExpression.java:371)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.expressions.IfExpression.applyConsequence(IfExpression.java:254)
at openwfe.org.engine.expressions.IfExpression.apply(IfExpression.java:144)
at openwfe.org.engine.expressions.raw.RawExpression.apply(RawExpression.java:371)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.expressions.SequentialFlowExpression.apply(SequentialFlowExpression.java:140)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:554)
at openwfe.org.engine.expressions.SequenceExpression.reply(SequenceExpression.java:130)
at openwfe.org.engine.expressions.state.NormalState.reply(NormalState.java:118)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.reply(SimpleExpressionPool.java:713)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.replyToParent(SimpleExpressionPool.java:644)
at openwfe.org.engine.expressions.AbstractFlowExpression.replyToParent(AbstractFlowExpression.java:479)
at openwfe.org.engine.expressions.AbstractFlowExpression.applyToParent(AbstractFlowExpression.java:494)
at openwfe.org.engine.expressions.LogExpression.apply(LogExpression.java:153)
at openwfe.org.engine.expressions.raw.RawExpression.apply(RawExpression.java:371)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.expressions.SequentialFlowExpression.apply(SequentialFlowExpression.java:140)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:554)
at openwfe.org.engine.expressions.SequenceExpression.reply(SequenceExpression.java:130)
at openwfe.org.engine.expressions.state.NormalState.reply(NormalState.java:118)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.reply(SimpleExpressionPool.java:713)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.replyToParent(SimpleExpressionPool.java:644)
at openwfe.org.engine.expressions.AbstractFlowExpression.replyToParent(AbstractFlowExpression.java:479)
at openwfe.org.engine.expressions.IfExpression.applyConsequence(IfExpression.java:260)
at openwfe.org.engine.expressions.IfExpression.apply(IfExpression.java:144)
at openwfe.org.engine.expressions.raw.RawExpression.apply(RawExpression.java:371)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.expressions.SequentialFlowExpression.apply(SequentialFlowExpression.java:140)
at openwfe.org.engine.expressions.state.NormalState.apply(NormalState.java:112)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.applyOrLaunch(SimpleExpressionPool.java:529)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:542)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.apply(SimpleExpressionPool.java:554)
at openwfe.org.engine.expressions.SequenceExpression.reply(SequenceExpression.java:130)
at openwfe.org.engine.expressions.state.NormalState.reply(NormalState.java:118)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.reply(SimpleExpressionPool.java:713)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.replyToParent(SimpleExpressionPool.java:644)
at openwfe.org.engine.expressions.AbstractFlowExpression.replyToParent(AbstractFlowExpression.java:479)
at openwfe.org.engine.expressions.OneOrZeroChildExpression.reply(OneOrZeroChildExpression.java:92)
at openwfe.org.engine.expressions.ParticipantExpression.reply(ParticipantExpression.java:655)
at openwfe.org.engine.expressions.state.NormalState.reply(NormalState.java:118)
at openwfe.org.engine.impl.expool.SimpleExpressionPool.reply(SimpleExpressionPool.java:713)
at openwfe.org.embed.impl.engine.AbstractEngine.reply(AbstractEngine.java:264)
at info.magnolia.module.workflow.WorkflowUtil.proceed(WorkflowUtil.java:169)
at info.magnolia.module.workflow.WorkflowUtil.proceed(WorkflowUtil.java:152)
at info.magnolia.module.workflow.WorkflowUtil.proceed(WorkflowUtil.java:129)
at info.magnolia.module.workflow.inbox.Inbox.proceed(Inbox.java:259)
at java.lang.reflect.Method.invoke(Method.java:597)
at info.magnolia.cms.servlets.MVCServletHandlerImpl.execute(MVCServletHandlerImpl.java:96)
at info.magnolia.cms.servlets.CommandBasedMVCServletHandler.execute(CommandBasedMVCServletHandler.java:66)
at info.magnolia.cms.servlets.MVCServlet.doPost(MVCServlet.java:111)
at org.springframework.orm.hibernate3.support.OpenSessionInViewFilter.doFilterInternal(OpenSessionInViewFilter.java:174)
at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:77)