Crisis on Infinite Earths
One of the best things about getting to know the owner of a comic shop is that you can read as many comics as you liked, especially the old ones. Comics span decades, so if you only read what is published during "your time," then you miss a lot.
As kids, my friends and I would read a ton of old comics. The owner was a fan himself, so maybe he just wanted to share some of the fun too. Or, maybe he just felt that there was no reason to just let the comics sit in a box. In any case, it was all gold to us. Most of the comics were actually pretty bad. Bad art. Silly stories. Etc. After a while, it became apparent that there was no consistency. The biggest culprit of this was DC.
In the old comics, Superman was infinitely powerful (still kinda is). Super-intelligence. Super-intuition. Telepathy. Hypnosis. Super-Amnesia-kisses (actually used in the movies too). At one point, Superman destroyed a universe with just a sneeze. It was becoming painful to read.
By the mid-80s, it was apparently becoming painful for DC too. Enter Crisis on Infinite Earths. This was DC's attempt to straighten everything out. They killed off some characters. They re-defined powers. This was good and bad. Some fan favorites were lost. Some characters were sloppily written back in. Later down the road, more continuity issues came up. In any case, this is a nice read for the history books. Plus, it used one of the most iconic comic covers of all time: Superman hold Supergirl's dead body. This would be used again.
DC would continue to complicate their timeline again and again. Another Earth. Another timeline. Another Crisis. A New 52.