Guide to LaTeX (4th Edition) (Tools and Techniques

402 Appendix C. Error Messages
Here the command \textbf has been mistakenly typed as \txetbf. During
the processing, L AT E X assumes that the user wants to invoke a T E X
command \txetbf. Since L AT E X does not know the T E X commands, it
passes this text on to T E X, which then decides that there is no such command
in its repertoire. The following error message is written to the
monitor:
! Undefined control sequence.
l.3 The last words appear in \txetbf
{bold face}.
?
The program stops at this point and waits for a response from the user.
This message can be understood even by beginners. It consists of an error
indicator that starts with an exclamation point !. Here the indicator is:
! Undefined control sequence, meaning that an unknown command
name (control sequence) was the cause of the error. Next comes a pair of
text lines, the first of which is prefixed with l.3, meaning that the error
occurred in ‘line 3’ of the input text. The error itself was encountered
at the last symbol printed in this upper line. The lower line shows the
continuation of the input line being processed when the error was found,
here the words {bold face}. Before continuing, T E X waits for a reaction
from the user, as indicated by the question mark in the last line of the
message.
Entering another ? and 〈return〉 as response produces the following
information:
Type to proceed, S to scroll future error messages,
R to run without stopping, Q to run quietly,
I to insert something, E to edit your file,
1 or ... or 9 to ignore next 1 to 9 tokens of input,
H for help, X to quit
?
This is a list of the possible user responses:
1. 〈return〉: simply typing the return key tells T E X to continue processing
after making an attempt to handle the error according to some
preprogrammed rules. In the case of an unknown command name,
the error treatment is to ignore it.
2. S scroll mode: T E X continues the processing, writing further error
messages to the monitor as they are encountered, but without stopping
for a user response. This is as though 〈return〉 were pressed
after all subsequent errors.
3. R run mode: T E X continues processing as with S, but does not even
stop as it would in scroll mode if the file named in an \input or
\include command is missing.