Borrowed a bike once, years ago.. ATV actually. The annual boys fishing/beer drinking and poker playing trip was coming around and me and my buddy are wishing for an ATV vs taking the long way around in the Jeep. Another friend, older and much more financial esstablished then either of us were back thenkindly offers up his "I never use it, please..take it out for a run and have a good time". Me and my buddy were of the "never borrow things" mind set but wanting to ride in on the trails, drinking beer etc (back in the day this was acceptable), we talked ourselves into it.
Off we go, get a few km into the trail and "cough, sputter stall" the bike is done. Looking at the fuel filter it is gummed up with what looks like mud, turns out it was rust from the rusting fuel tank of the hardly used ATV.
Long story short end up getting towed out of the woods twice, replacing the filter and fuel pump, epoxy coating the rusted fuel tank, paying someone to cut the fuel tank open to remove the epoxy from the screens on the end of the fuel pick ups and eventually getting the bike back to the owner. Both of us were out of pocket some $ for parts and repair and a bottle of whiskey to the guy who drove us out of the woods with his pickup. I think the owner of the ATV feeling badly repaid us once found out the trouble we had.
Lesson learned. Don't borrow things.