In many problems the output is a list of n elements which must be output as follows:
X1 X2 X3 ... Xn
(each element being separated from the next one by a space character)
Depending on the language, producing the required output can be painful (e.g. easy in python/ruby, painful in C/Java). The easiest way to code this is to add a space each time an element is output in the loop:
for(i=0;i<n;i++) print(X[i] + " ");
Obviously this kind of code adds an additional space at the end.
Obtaining the strict output is not difficult but requires time and more code to write (code which is not fun to write and which brings nothing to the user).
This is penalizing in CoC (in all modes : speed, size and even reverse) and boring in practice puzzles.
A simple solution would be to ignore trailing spaces when checking the solution. This is very simple to implement on the server and would make a lot of people happy