Yes - once on an internal flight in Pakistan between Islamabad & Lahore....
Flight was diverted to Multan due to massive thunderstorms in the Lahore area - we were in an F28 - and prior to landing in Multan we went through some dreadful turbulence - at one point we dropped like a stone for what seemed like 30 seconds. I'm sure it was more like 2 seconds but that is the only time I've ever felt slightly concerned whilst flying....