Well took a C172 over the Cairngorms above cloud for maint.
I had done the usual limited engine inspection that you can do with a C172 ie shone a torch down the oil filler cap.
When waiting the Engineer came through with the oil cooler pipe which had been chaffed down and was now weeping oil. If that had gone at FL90 over the Cairngorms I would have been stuffed.
This was the beginning of my adverstion of flying singles at night and out of sight of the ground.