I recall a 28 buy-in downswing at $25 @ Party about a month into playing cash games.
Basically I won $700 in about two weeks, then lost it back over the next 8 weeks...obv. I simply wasn't good enough at cash games or equipped to deal with tilt at that time.
It did have the side effect if making sure I didn't move up until I had 40x BI for the next level from then on, so I've basically had zero risk of ruin throughout my playing career.
I would say that I've had three 15BI dowswings in the 4 years since. Last one being this January and the one before that being February 2007.
Tilt was a major factor each time.
In maybe 100k hands of shortstacking I've yet to see a downswing worse than 4 (full) buy-ins for obvious reasons.