From an Astronomy point of view: when I pointed my 60mm scope at the "easy" double Castor in 1968 and failed to split. It turned out that the Nortons Atlas had 'old' information and the separation at that time was much closer than listed. Now of course, it is easy to split.
On the same evening, I felt similarly duded when I pointed the 60mm scope at the Virgo cluster of galaxies. Nortons showed the area congested with deep-sky objects, but, I saw nothing from my light-polluted backyard.