Learn English – Noun for person being observed

Sympathetic is the first word that comes to mind: it can mean both feeling sympathy and causing sympathy. Here are the two relevant definitions from the Oxford English Dictionary:

Tending to elicit sympathy or to induce a feeling of rapport; also loosely, pleasant, likeable. (Example: 1900 Beerbohm in Sat. Rev. 10 Mar. 295/2 "The true Don Juan‥is‥not a ‘sympathetic’ part.")

Feeling or susceptible of sympathy; sharing or affected by the feelings of another or others; having a fellow-feeling; sympathizing, compassionate. (Example: 1867 Dickens Let. 6 Mar. (1999) XI. 327 "An unusually tender and sympathetic audience.")

From what I understand, "hamartia" can be used more generally to refer to a tragic or fatal flaw. There is also a medical definition that seems to be about abnormal tissue growth (though it is not a tumour). So you could probably use "hamartia" when referring to a fatal flaw, but I suspect most people who have not studied medicine or Greek tragedies will not get the reference.

In conclusion: Don't do it (unless you want to completely confuse your audience).

I think example will work (since you insisted):

The web application's hamartia was failing to sanitize input on the user sign-in form. This allowed the database to be compromised and credit card numbers to be stolen.