[latex] Getting the error "Missing $ inserted" in LaTeX

I had this problem too. I solved it by removing the unnecessary blank line between equation tags. This gives the error:

\begin{equation}
P(\underline{\hat{X}} | \underline{Y}) = ...

\end{equation}

while this code compiles succesfully:

\begin{equation}
P(\underline{\hat{X}} | \underline{Y}) = ...
\end{equation}