South of Odessa, NE on May 29th, 2008. Got in great position to watch a supercell move east... dual wall clouds, great structure... not producing, not producing... but as the storm got nearly overhead, the south winds increased dramatically, the southern wall cloud REALLY tightened up, and all of the sudden there was this mass of swirling dirt in a field maybe a quarter of a mile from me... while other chasers were heading north towards the storm... I was fleeing south on the Odessa road to reposition and make sure I was safe.
Another good one for me would be near Attica, KS, on April 26th, 2009. There was a tornado warning to the southwest of Wichita (Barber County I think), but very near that tornado warning was a ferocious little bow echo. The precipitation for the echo was pure white and racing towards the road I was on. Needless to say I punched east with no delay... and then drove away from the storm that produced the Viola tornadoes. D'oh...