SlipBehavior.Slip sólo se admite en un ParallelTimeline raíz que no invierta, acelere, desacelere ni tenga un RepeatBehavior especificado como Duration.
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English version
SlipBehavior.Slip is supported only on root ParallelTimelines that do not reverse, accelerate, decelerate, or have a RepeatBehavior specified as a Duration.
.Net Framework