Top 20 LaTeX Errors and How to Fix Them

Every error here has been made by every LaTeX user. The fix is usually one line.

These twenty LaTeX errors account for nearly every failed compile you’ll ever see, and each fix is usually one line. The skill isn’t avoiding them, everyone triggers them, it’s recognizing the message and knowing the reflex fix.

1. Undefined control sequence

! Undefined control sequence.
l.15 \bgin{equation}

Cause: Typo in a command name. Here, \bgin should be \begin.

Fix: Check spelling. Check that the package providing the command is loaded.

2. Missing $ inserted

! Missing $ inserted.
l.22 x^2

Cause: Math syntax outside math mode.

Fix: Wrap in $...$ or <!--M0-->.

3. Missing } inserted / Extra }

! Missing } inserted.

Cause: Unmatched braces.

Fix: Count your braces. Every { needs a }. Use editor brace matching.

4. Environment ended by \end{wrong}

! LaTeX Error: \begin{equation} ended by \end{align}.

Cause: Mismatched \begin/\end environment names.

Fix: Make them match.

5. File not found

! LaTeX Error: File `fancyhdr.sty' not found.

Cause: Package not installed.

Fix: tlmgr install fancyhdr (TeX Live) or let MiKTeX auto-install.

6. Too many }’s

! Too many }'s.
l.30 \frac{a}{b}}

Cause: Extra closing brace.

Fix: Remove the extra }.

7. Misplaced alignment tab character &

Cause: Using & outside a tabular or alignment environment.

Fix: Use \& in text, or put & only inside tabular, align, etc.

8. Extra alignment tab has been changed to \cr

Cause: More & columns than the table specification allows.

Fix: Add a column to your specifier or remove the extra &.

9. Font not found (xelatex)

! fontspec error: "font-not-found"
! The font "STIX Two Text" cannot be found.

Cause: Font not installed on your system.

Fix: Install the font or change \setmainfont to an available font.

10. Overfull \hbox

Overfull \hbox (15.3pt too wide) in paragraph at lines 40--45

Cause: Content extends past the margin.

Fix: Rephrase text, use \sloppy, add hyphenation hints, or break long URLs with \url.

11. Underfull \hbox

Cause: A line has too much white space (badness > 1000).

Fix: Usually harmless. Rephrase if the gap is visually noticeable.

12. Reference undefined

Cause: Label doesn’t exist, or you only compiled once.

Fix: Compile twice. Check that \label{sec:foo} exists.

13. Citation undefined

Cause: BibTeX/Biber hasn’t been run, or the key is wrong.

Fix: Run bibtex/biber, then compile twice more.

14. Package hyperref Warning: Token not allowed

Cause: Math or special commands in section titles with bookmarks.

Fix: Use \texorpdfstring: \section{The \texorpdfstring{\LaTeX}{LaTeX} Book}.

15. Float too large for page

Cause: Figure/table is taller than the text area.

Fix: Reduce image size or use \includegraphics[height=0.8\textheight].

16. Option clash for package

! LaTeX Error: Option clash for package xcolor.

Cause: Package loaded twice with different options.

Fix: Load the package once (with all options), or use \PassOptionsToPackage before \documentclass.

17. Command already defined

Cause: \newcommand on a name that already exists.

Fix: Use \renewcommand if intentional, or choose a different name.

18. Dimension too large

Cause: A length exceeds TeX’s maximum (~5.75 meters).

Fix: Check for infinite loops in length calculations or very large scaling factors.

19. TeX capacity exceeded

Cause: Infinite recursion or extremely complex macros.

Fix: Check for recursive \newcommand definitions. Simplify.

20. Paragraph ended before command was complete

Cause: Blank line inside a command argument.

Fix: Remove the blank line or use % at the end of lines to prevent paragraph breaks.

Tip: When debugging, always fix the first error and recompile. Later errors are often cascading effects of the first one. Don’t try to fix all errors at once.

Errors fixed, back to writing: the symbol quick reference and package recommendations are the other two appendices built for mid-work consultation.

Quick answers to common questions: