It's airshow season! While you're out there show the same sort of respect you would (or at least should) at car shows. That means no touching! While leaning against someone's '69 Camaro fender might scuff the paint and form a little dent, leaning against the leading edge of an airplane wing can have more dire consequences then a curse-filled chewing out. Here's why.
Topshot by John Murphy of the Airplane Owner's and Pilots Association, used under Creative Commons license. Wing diagram by Andrew Fry via Wikipedia, used under Creative Commons license.
Aircraft wings work by manipulating the airflow in a precise way - how that airflow is manipulated can be modified for certain performance characteristics, but generally all wings work by being precision-crafted smooth planar surfaces. Disruption of that airflow by dents or dings can spoil the airflow leading to loss of performance and/or lift.
Diagram by nomenclaturo.com
Denting or forming depressions on the leading edge of the wing is pretty easy to do for many if not most light aircraft. The upper shape of the wing is formed by thin aluminum skin panels joined to wing ribs as illustrated above. When I say thin aluminum skin panels, I mean potentially about the same thickness and strength as your typical soda can. The ribs give the wing most of its structural strength, but the leading edge of the wing in between these ribs is especially vulnerable to impacts or even relatively soft forces that can deform the leading edge and spoil smooth airflow, leading to performance derogation and potentially compromised safety.
That's why it's important to not lean against an aircraft's leading edge. Many of the aircraft you see at airshows are expensive antiques costing as little as a brand new Focus ST or FR-S, or as much as a new 458 Italia (or if it's a war-provenanced P-51, a LaFerrari) . Unless the aircraft happens to be "based" right at that field, almost all of them have been flown in for the airshow. Don't make flying back home any more dangerous and frustrating than it has to be - look with your eyes and only touch and step on whatever areas the pilot or caretaker designates for you.