[Tex/LaTex] What are the most common mistakes that beginners of (La)TeX and Friends make

Question

I want to collect the most common mistakes, misconceptions, pitfalls, etc that (La)TeX and Friends users make. Please one answer for each mistake, misconception, pitfall, etc.

I am only interested in more technical, objective cases rather than psychological, social, subjective cases.

From this complete infinite list of technical and objective cases, we can have a reference how to teach newbies much better directly or indirectly via writing a good book.

The technical and object cases are, for example, as follows.

• Beginners sometimes use $\huge E=mc^2$ with the hope they will get a huge formula. It actually does not produce the expected result. The correct way is \huge $E=mc^2$.
• Beginner sometimes use longtable inside table because they assume longtable is the longer version of tabular which is able to be sandwiched in table.
• etc.

Answer 1

The most common mistake is spending too long on TeX coding and not getting the document written.