This error appears when the alignment character '&' is used incorrectly. The alignment character & is used to align elements in specific environments, such as matrix, align, table etc.

Common Causes of Error

Not writing \&:

When you want to use an ampersand & as a text character in your writing, such as in a title, you must write it as \&. If you fail to do this, you will get an error as shown below

The company is named `Michael & Sons'

To correct this error, change & to \&.

Matrix macro used instead of amsmath:matrix:

In the basic LaTeX distribution, there is a macro called matrix. This is often used incorrectly when what you really want to use is the environment matrix supplied by the amsmath package. If you forget to load the amsmath package and simply write

$$
\begin{matrix} 
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9
\end{matrix}
$$

then a misplaced alignment tab error message will appear. This is because this is not the correct way to use the matrix macro supplied by the LaTeX distribution. In order to resolve this problem, simply load the amsmath package.

% In your preamble (before \begin{document})
\usepackage{amsmath}

% In your document

$$
\begin{matrix} 
1 & 2 & 3 \\
4 & 5 & 6 \\
7 & 8 & 9
\end{matrix}
$$