When you run a routine in the terminal and an error occurs in the program, if you have not set the error trap properly, the program will enter debug mode as shown below.
I came up with a challenge for myself to come up with a way to make a variable watch itself for a certain value and do something when it hits that value without having to check it every time something touches it. Basically, a way to say "if at any point during the execution of this code, if x = 0 (or whatever the condition is) do this thing." The class I ended up with watches a %Status: