The structure of TeX error messages

TeX's error messages are reminiscent of the time when TeX itself was conceived (the 1970s): they're not terribly user-friendly, though they do contain all the information that TeX can offer, usually in a pretty concise way.

TeX's error reports all have the same structure:

The error message will relate to the TeX condition that is causing a problem. Sadly, in the case of complex macro packages such as LaTeX, the underlying TeX problem may be superficially difficult to relate to the actual problem in the "higher-level" macros. Many LaTeX-detected problems manifest themselves as 'generic' errors, with error text provided by LaTeX itself (or by a LaTeX class or package).

The context of the error is a stylised representation of what TeX was doing at the point that it detected the error. As noted in approaching errors, a macro package can tell TeX how much context to display, and the user may need to undo what the package has done. Each line of context is split at the point of the error; if the error actually occurred in a macro called from the present line, the break is at the point of the call. (If the called object is defined with arguments, the "point of call" is after all the arguments have been scanned.) For example: 

\blah and so on
produces the error report 
! Undefined control sequence.
l.4 \blah
          and so on
while: 
\newcommand{\blah}[1]{\bleah #1}
\blah{to you}, folks
produces the error report 
! Undefined control sequence.
\blah #1->\bleah 
                 #1
l.5 \blah{to you}
                 , folks
If the argument itself is in error, we will see things such as 
\newcommand{\blah}[1]{#1 to you}
\blah{\bleah}, folks
producing 
! Undefined control sequence.
<argument> \bleah 
                  
l.5 \blah{\bleah}
                 , folks

The prompt accepts single-character commands: the list of what's available may be had by typing ?. One immediately valuable command is h, which gives you an expansion of TeXs original précis message, sometimes accompanied by a hint on what to do to work round the problem in the short term. If you simply type 'return' (or whatever else your system uses to signal the end of a line) at the prompt, TeX will attempt to carry on (often with rather little success).