PIS

From Rumblepedia

PIS stands for Plot Induced Stupidity.

PIS is when the writer ignores or forgets a character's abilities or simply writes them as "dumb", usually with no cause or reasoning beyond simply needing them to for the sake of their plot to work; hence 'Plot' induced stupidity.

And in turn, said character performs at a significantly lower level compared to when he/she is written properly.

A frequent example of PIS is the Flash. Without PIS, the Flash could triumph over most of his foes in a few panels, rendering a comic series pointless and uninteresting.