# Underfull \hbox, undefined references, and labels may have changed. Appearing after already working correctly

My document was compiling correctly until I started copy and pasting stuff into another document. Now I get underfull \hbox, undefined references, and labels may have changed warnings. All of my \cite{key} references are now undefined.

\documentclass[a4paper, 11pt]{article}
\usepackage[utf8]{inputenc}
\usepackage[a4paper, top=1.5cm,bottom=1.5cm, left=2cm, right=2cm, marginparwidth=1.75cm]{geometry}
\begin{document}

Before discussing the details of how the idea CP symmetry came to be, and how it was eventually broken, a brief elaboration of charge and parity conjugation is in order. Many are familiar with the concept of charge, electric charges are dealt with frequently. Charge is thought to be a fundamental property of matter, and charge conjugation is switching the charge of a piece of matter to negative of its original value. What this does is change matter to anti-matter, which is defined to be nearly equivalent to matter but with opposite charge.\cite{Scollia}  For example, conjugating the charge of a proton gives an anti-proton, and conjugating the charge of an electron gives you a positron. Parity conjugation is simply switching the spatial coordinates of something. If an object is located at a point $P=(1,3,5)$, conjugating its parity leaves it at a point $Q=(-1,-3,-5)$.\cite{Scollia} CP symmetry means that the laws of physics should remain the same, or be symmetric, when the two are conjugated separately. Physicists seem to like symmetry in theories, but these symmetries have certainly tested their patience. 

\begin{thebibliography}{9}
\bibitem{Pandolfi} Pandolfi, Stephania. "CERN CASTs new limits on dark matter." \textit{CERN}. 1 May, 2017. \linebreak \indent https://home.cern/about/updates/2017/05/cern-casts-new-limits-dark-matter
\bibitem{Chu}  Chu, Jennifer. "Team simulates a magnetar to seek dark matter particle." \textit{phys.org}. 7 October, 2016. \linebreak \indent https://phys.org/news/2016-10-team-simulates-magnetar-dark-particle.html\#nRlv
\bibitem{Scollia} Scollia, Gabriella. "The Mystery of CP Violation." \textit{MIT Annual 2006}. 2006. \linebreak \indent http://web.mit.edu/physics/news/physicsatmit/physicsatmit\_06\_sciollafeature.pdf
\bibitem{Nevis} "CP Violation." \textit{Nevis Labs}. \linebreak \indent https://www.nevis.columbia.edu/daedalus/motiv/cp.html
\bibitem{IAXO} "IAXO, Physics" \textit{CERN}. \linebreak \indent http://iaxo.web.cern.ch/content/physics
\bibitem{Wall} Wall, Mike. "What is Dark Matter? Prime Candidate Gets Profiled." \textit{space.com}. 2 November, 2016. \linebreak \indent https://www.space.com/34595-dark-matter-search-axion-mass.html
\bibitem{Wesson} Wesson, Paul. "Fundamental Unsolved Problems in Physics and Astrophysics." \textit{Calphysics Institute}. \linebreak \indent http://www.calphysics.org/problems.pdf
\end{thebibliography}

\end{document}


I'm pretty worried because I was finished and now nothing will compile. I keep getting a "MikTex compiler driver did not succeed."

I copy and pasted the whole thing into a new file and it worked again. Now I'm even more confused.

• This, as posted cannot compile. There is no document environment. Can you provide a minimal working example? en.wikibooks.org/wiki/LaTeX/How_to_get_help – Johannes_B Apr 30 '18 at 4:00
• I changed your code to be compilable. As you can see, it works perfectly fine (as you already noted). I guess deleting the auxiliary files will help in your case (as often). Why are you using \linebreak? You probably want \newline or \par` instead? – Johannes_B Apr 30 '18 at 4:27
• I only knew about linebreak, I'll start using the other ones. Thanks – roshoka Apr 30 '18 at 13:13