I kind of hate this expression, because it suggests there's no such thing as common sense.
Leaving a purse full of valuables unattended is a dumb thing to do. If we can't call a spade a spade, then we are lost. I'm sure all of us have done something stupid at one time or another. I'm willing to bet that most folks responding here have, at one time or another, left items of value behind by accident.
But then I also disagree with this poster:
Parks are no safer(theft) than anywhere else.
Because this hasn't been our experience. We try to be careful with valuables, but over a great many trips to WDW with extended family, we never had anything (non-valuable) taken from a stroller.
No, I take that back, a seagull swooped in to steal our popcorn, and a crafty Universal Studios squirrel stole our french fries! In both cases, we were standing next to the stroller, having momentarily placed each food in the cup holder.
And quite a few times, we've accidentally left a bag or purse behind. We've also accidentally left stuff in the ride storage compartments (like the under the seat on Soarin'.) On BTMRR, my cell phone came out of my back pocket, and I didn't notice until I went to use it. Another time, a member of my family had their wallet fall apart on a plane. She didn't realize that $1000 worth of WDW park tickets, plus $500 had slid out of the wallet, back several rows along the floor. Honest folks returned our stuff every time.
Overwhelmingly, Disney and Disney guests have had an AMAZING track record of finding our stuff and returning it to us fully in tact. CM's are vigilant and honest. Guests have often been amazingly generous with their vacation time, even chasing after us with our stuff! Many times, we've also returned the favor.
For a great many years, WDW has often brought out the best in people when it comes to situations like this. Long lines may make folks grumpy, but they also go to WDW to experience happy endings. Finding your lost wallet in tact is among the happiest of endings!