If I use the [hang] format of the titlesec package (using a quite recent version of titlesec, namely 2016/03/15 v2.10.1, and TeX Live 2015), the section is not numbered. It seems that the [hang] option disables the numbering.
MWE (taken from https://tex.stackexchange.com/a/109866/24664, the image there shows my expected outcome):
\documentclass{article}
\usepackage{tgtermes,tgheros} % use your preferred ones for the fonts
\usepackage{lipsum} % this is just to produce some text
\usepackage{titlesec}
\titleformat{\section}[block]
{\fontsize{12}{15}\bfseries\sffamily\filcenter}
{\thesection}
{1em}
{\MakeUppercase}
\titleformat{\subsection}[hang]
{\fontsize{12}{15}\bfseries\sffamily}
{\thesubsection}
{1em}
{}
\begin{document}
\lipsum[1]
\section{A section title}
\lipsum[2]
\subsection{A subsection title}
\lipsum[3]
\end{document}
Is this a bug in the titlesec package?
titlesec 2016/03/21 v2.10.2. – Alenanno May 25 '16 at 14:452.10.2from CTAN and got the expected output. Thank you! – gerw May 25 '16 at 15:13