\cr& signs that separate the columns of a tabular; so the error
message
! Extra alignment tab has been changed to \cr
could arise from a simple typo, such as:
\begin{tabular}{ll}
hello & there & jim \\
goodbye & now
\end{tabular}
where the second & in the first line of the table is more than the
two-column ll column specification can cope with. In this
case, an extra “l” in that solves the problem. (If you
continue from the error in this case, “jim” will be moved
to a row of his own.) Another simple typo that can provoke the error
is:
\begin{tabular}{ll}
hello & there
goodbye & now
\end{tabular}
where the ‘\\’ has been missed from the first line of the table.
In this case, if you continue from the error, you will find that
LaTeX has made a table equivalent to:
\begin{tabular}{ll}
hello & there goodbye\\
now
\end{tabular}
(with the second line of the table having only one cell).
Rather more difficult to spot is the occurrence of the error when you’re using alignment instructions in a “p” column:
\usepackage{array}
...
\begin{tabular}{l>{\raggedright}p{2in}}
here & we are again \\
happy & as can be
\end{tabular}
the problem here (as explained in
tabular cell alignment) is that the
\raggedright command in the column specification has overwritten
tabular’s definition of \\, so that
“happy” appears in a new line of the second column, and the
following & appears to LaTeX just like the second
& in the first example above.
\tabularnewline explicitly, or use the \RBS trick described
there.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=altabcr