I was trying to write a letter in LaTeX the other day:
\documentclass{letter}
\address{Nowheresville}
\signature{Me}
\begin{document}
\begin{letter}
\opening{To Whom It May Concern:}
Hello, there.
\closing{Sincerely,}
\end{letter}
\end{document}
This led to the following two errors, which shed little light on the situation:
! LaTeX Error: There's no line here to end.
See the LaTeX manual or LaTeX Companion for explanation.
Type H for immediate help.
...
l.10 \opening{To Whom It May Concern:}
and (on a different example)
! Incomplete \iffalse; all text was ignored after line 66.
\fi
l.16 \end{letter}
Runaway text?
\@mlabel{}{\unhbox \voidb@x \ignorespaces \global \let
The problem, as it was gently explained to me, is I had omitted the second mandatory argument of \begin{letter}, which is the address of the recipient. The following is correct:
\documentclass{letter}
\address{Nowheresville}
\signature{Me}
\begin{document}
\begin{letter}{Foo Corp.}
\opening{To Whom It May Concern:}
Hello, there.
\closing{Sincerely,}
\end{letter}
\end{document}
[UPDATE] I just realized that the reason I got so confused about this is that I was working off a previous business letter that was formatted like:
\begin{document}
\begin{letter}
{
Foo Corp. \\
... \\
ATTN: Warranty Dept.}
...
I’m not sure if I intended it to be the case (probably not), but LaTeX picked up the braces around the address as the argument to letter. When I used this as the template for a personal letter and deleted the address, all hell broke loose.
Procrastiblog 
Thanks for posting. I was having a similar errors (using a different class) and my solution was the same.
Comment by fs — July 8, 2008 @ 1:22 pm
This blog post is up for nearly a full year, and then it suddenly receives two responses within four days of each other. Go figure.
Anyhow, thank you for sharing this solution. I am using the LyX editor on OS X, and you described my problem (and solution) perfectly. Thanks again.
Comment by Rockwell — July 12, 2008 @ 7:46 pm
As a follow-up to my own comment #2, I just came across an alternate way of defining a letter, using the \letter command. It formats the letter slightly differently, and it appears that you can only include one such command per document.
\begin{document}
\address{Sender’s address}
\letter{Recipient’s Address}
\opening{Dear Recipient,}
Letter content goes here
\closing{Sincerely,\\
The Procrastiblog}
\end{document}
Comment by Rockwell — July 12, 2008 @ 7:58 pm
fs, Rockwell, Glad to be helpful.
Rockwell, Hmm… A
letterdocument class, environment, and command? I need to look into this…Comment by Chris — July 12, 2008 @ 11:26 pm