Subsetting data and using the wrong indices

Read in files with numerical names in the wrong sort order (1,11,110,…)

Duplicated code

Duplicated code where one duplication is a little bit different than the other

Detecting the correct text coding of a file

Global variables

Script files with names which have absolutely nothing to do with their content

Variable names which have absolutely nothing to do with their purpose

Quick modifications of recursive functions

Time zones

Reading/Writing contents of PDF files

Assuming that someone will create required system setup that you need in your code (do it yourself!)

…

Improve CPP performance by avoiding the heap