When you use a magic value in a particular field (such as a text field) in order to signify something else. For example, if you use a magic text pattern (like ‘XXX’) to signify the last record.
This can kill you because (a) what if the user types in “XXX” into that field? And (b) if you’re storing the record as an explicit marker, what happens if your magic record goes away?
Better off either creating some sort of conversion protocol to allow out-of-band information to reside within the text (like the & escape character in HTML), or to create a separate control channel to contain control data. And if you’re using the symbol as an end of array marker, you’re much better off using the built-in OS routines (end of file marker, end of database marker, etc) to determine you’re at the end of the list.