Is it bad to put the \ to the end of the comment line?

If the end of the comment is , the second line will be included in the comment.
Therefore, the second line of the instruction is not executed.
I think the end escape code is causing.

// print out the state of the button:
Serial.println(buttonState);

Yes it is bad to do that.

This is intended and documented behavior for "" at the end of the line in a C program. It acts as a "line continuation marker", essentially deleting the newline.
http://en.wikibooks.org/wiki/C++_Programming/Code/Style_Conventions/Comments

Exceedingly bad. It could lead someone to believe that the following line of code is not commented out, when it is.

Everyone, Thank you for your polite comments.
I was also examined.
Character code $5C is change of assignment was observed at ISO646, in Japan backslash is in the YEN symbol.
YEN is the currency of Japan.
I find it is necessary attention to this code.
Thank you!!

westfw:
This is intended and documented behavior for "" at the end of the line in a C program. It acts as a "line continuation marker", essentially deleting the newline.
C++ Programming/Code/Style Conventions/Comments - Wikibooks, open books for an open world

Unless you are using Arduino 1.6.0 where including continuation character at the end of comment line is ignored as it should be.
There are standards, Arduino and wiki. One should be free to use the correct definition.
Still a bad practice irregardless how it works.

Vaclav:
Unless you are using Arduino 1.6.0 where including continuation character at the end of comment line is ignored as it should be.

Each instance of a backslash character () immediately followed by a new-line
character is deleted, splicing physical source lines to form logical source lines.
Only the last backslash on any physical source line shall be eligible for being part
of such a splice. A source file that is not empty shall end in a new-line character,
which shall not be immediately preceded by a backslash character before any such
splicing takes place.

Vaclav:
Unless you are using Arduino 1.6.0 where including continuation character at the end of comment line is ignored as it should be.
There are standards, Arduino and wiki.

Quote your source Vaclav, referencing the C++ standard.

The C preprocessor has always had some ambiguities with respect to “order of processing.” In this case comment processing vs EOL processing. In Real Life, we had quite a “shock” when the gcc people changed the order of stringification vs concatenation - you couldn’t find anything to say that one way was right and the other was wrong, but thousands of files wouldn’t compile any more, and it wasn’t possible to fix the macros that were involved.