Alignment tab changed to \cr

This is an error you may encounter in LaTeX when a tabular environment is being processed. "Alignment tabs" are the & 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 - an extra "l" in that solves the problem. (As a result of the error, "jim" will be moved to a row of his own.)

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.

Get rid of the error in the way described in tabular cell alignment - either use \tabularnewline explicitly, or use the \RBS trick described there.

array.sty
Distributed as part of macros/latex/required/tools (zip, browse)

This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=altabcr