Glad you caught it before it caused problems! I am among the nuts that climb up top to sweep off the slideout when breaking camp. I am worried that acorns or twigs will damage the wiper seal, or will cause the damage you saw. When I winterized, I put the slide in and cleaned and inspected the top of the slide. I had to do some acrobatics but I was glad I took the time, if only for the peace of mind. I did discover that the seal was twisted - half of the seal was pointing outside and half pointed inside. There was a little gap where the twist was. Since the seal is pretty new and flexible I think it will "remember" the correction.
2011 Sabre Silhouette 260BHS
2006 F-250 6.OL