I found out today with 4.1 that if you have trace(so far only tested with MultiLineTraceByChannel and SingleLineTraceByChannel) node on a event tick line, and if your trace distance is either too short, or after moving around a while hits nothing in range, your control will freeze since it will never return when that happens.
Solution? Make a big enough box in your 3D package with normal facing inward, assign a alpha material that makes it transparent, but visible in game without casting shadow. That should do it before Epic give a fix update.
(remember to set your trace line long enough.)