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

C.3.2 L AT E X package errors
C.3. List of L AT E X error messages 421
The special programming commands for handling class and package files
(Appendix D) have their own set of error messages. If one of these features
issues a serious error, there is often little the user can do about it other
than report the problem to the author of the file. On the other hand,
some errors are due to the improper use of the file or of the options that
it provides.
Often classes and packages contain their own error or warning messages
with text and meanings that are peculiar to them. These are indicated
as such, along with the name of the class or package that issued
them. For example, the package mypack could print the error
Package mypack Error: cannot mix options ‘good’
(mypack) and ‘bad’.
A help message should also be available when H is typed. Obviously,
such error (and warning) messages cannot be explained here, since they
depend entirely on the package in question.
! LaTeX Error: \LoadClass in package file.
A package file has called \LoadClass, which it is not allowed to do. A
class file can only be loaded from another class file.
! LaTeX Error: Option clash for package ....
The specified package has been requested a second time with a different
set of options. A package file will only be loaded once and a second
attempt will be ignored. Thus, if two \usepackage or \RequirePackage
commands load the same package with different options, there is a conflict.
Try to arrange for a consistent set. Typing H for help after this
message will print out the two sets of options.
! LaTeX Error: \RequirePackage or \LoadClass in Options Section.
These two commands may not appear in the definition of a class or
package option made with the \DeclareOption command. Instead, the
option should set some flag or other indicator that is later tested before
calling the command in question.
! LaTeX Error: This file needs format ‘...’ but this is ‘...’.
The \NeedsTeXFormat command specifies a different format from the
one being used. The format is the prestored set of instructions that
determine what type of T E X is being run. For L AT E X 2ε, the format name
specified must be LaTeX2e. This file cannot be processed at all with the
current format in use.