Error Fixing
You might wonder why, if errors can be found automatically, they can't also be fixed automatically. The answer is that they could, but often it is more useful to identify the location of the error visually, as the error itself can point to other problems in the list that might otherwise go unnoticed. For instance, look at this section of a tab-indented list:
A
AB
AC
AD
AE
B
BA
BB
BC
The second line has more than 1 indent than its previous line. If you load this section of list into many programs, lines AB-AE would simply disappear. It would be quite easy to fix the problem automatically by removing one indent from the second line, but that would turn 'AB' into a hierarchical parent to the following three lines, which may not be what we want:
A
AB
AC
AD
AE
B
BA
BB
BC
In all likelihood, the lines AB-AE have just been mistakenly indented by one too many tabs, but it is only by actually looking that we can tell that for sure.