In passing from the code
\item $\Sp^\Sigma(\C,K)$ has a left proper, cellular model category
structure, called the \textbf{projective model structure}, where a map
$f\colon X\rightarrow Y$ in $\Sp^\Sigma(\C)$ is a weak
equivalence (resp., a fibration), if $f_n\colon X_n\rightarrow Y_n$ is
a weak equivalence (resp., a fibration) in $\C$, for all $n\in\bbN$.
to the code
\item $\Sp^\Sigma(\C,K)$ has a left proper, cellular model category
structure, called the \textbf{projective model structure}, where a map
$f\colon X\rightarrow Y$ in $\Sp^\Sigma(\C,K)$ is a weak % ← added ",K"
equivalence (resp., a fibration), if $f_n\colon X_n\rightarrow Y_n$ is
a weak equivalence (resp., a fibration) in $\C$, for all $n\in\bbN$.
where the only difference is in the marked K, I get the following Latex error at line 1, when trying to compile my document:
! \OML/cmm/m/it/10.95 Y$ \OT1/cmr/m/it/10.95 in $[][]\OT1/cmr/m/n/10.95 (\U/eus/m/n/10.95 C\OML/cmm/m/it/10.95 ; K\OT1/cmr/m/n/10.95 )
This makes absolutely no sense to me and I can not find much help on-line. Note also that, if I leave the problematic piece of code unaltered and keep on typing other stuff, I have no problem in compiling (even when I add a lot of code).
Upon seeing some comments, let me try to clarify the problem better. The following code uses all the packages I am using and compiles with no problems
\documentclass[11pt]{amsart}
\usepackage[utf8]{inputenc} % to allow unicode in source
\pagestyle{plain}
%% AMS and other general math packages:
\usepackage{amsthm}
\usepackage{amsfonts}
\usepackage{amsmath}
\usepackage{amssymb}
\let\amssquare\square
\usepackage{dsfont}
\usepackage{marvosym}
\usepackage{mathpartir}
\usepackage{mathtools} % for \mathclap, used to center extra-wide diagrams.
\usepackage{stmaryrd}
%% General style packages:
\usepackage{xcolor}
\definecolor{darkgreen}{rgb}{0,0.45,0}
\definecolor{darkred}{rgb}{0.75,0,0}
\definecolor{darkblue}{rgb}{0,0,0.6}
\usepackage[colorlinks,citecolor=darkgreen,linkcolor=darkred,urlcolor=darkblue]{hyperref}
\usepackage{breakurl}
\usepackage[mathscr]{eucal}
\usepackage{enumerate} % for customising enumerated listsEDIT:
\usepackage[capitalize]{cleveref}
%\parskip = 0.1in
%\parindent = 0.0in
%% For syntax of type theory:
\usepackage{mathpartir}
%% Graphics and diagrams packages:
\usepackage{tikz}
\usetikzlibrary{arrows}
\usepackage[all]{xy}
\xyoption{2cell}
\xyoption{curve}
\UseTwocells
\newcommand{\Sp}{\mathsf{Sp}}
\newcommand{\C}{\mathscr{C}}
\newcommand{\bbN}{\mathbb{N}}
\begin{document}
\begin{itemize}
\item $\Sp^\Sigma(\C,K)$ has a left proper, cellular model category structure, called the \textbf{projective model structure}, where a map $f\colon X\rightarrow Y$ in $\Sp^\Sigma(\C,K)$ is a weak equivalence (resp., a fibration), if $f_n\colon X_n\rightarrow Y_n$ is a weak equivalence (resp., a fibration) in $\C$, for all $n\in\bbN$.
\end{itemize}
\end{document}
This is the exact same code I am using in my problematic file and that does not allow me to compile. If, in my problematic file, I replace the second occurrence of
$\Sp^\Sigma(\C,K)$
with
$\Sp^\Sigma(\C)$
I do not generate any error. Here is a screenshot of the error I get in texmaker upon compiling. (The command \end{document} is present in the file, I just could not screenshot it). Again, the only thing that causes texmaker not to compile is adding that extra ,K. I am unable to get the error from the log file, as was suggested in the comment.
{}(not quote (>) section so line ends preserved!as an error. In which case your best bet here would be to just reword the text and/or do something else to fix the overfull box, or else increase the line width of error messages so that the line no longer starts with a!!in the warning message. But thanks to your answer I know why there's a!in the error message :-)