Flag: the_user_checks_the_sourcecode
The challenge flavortext says "One is not like the others..." and links to an HTML file containing a number of flags.
Viewing the source for the page, we see that one entry has been styled not to be displayed:
<a>flag{the_current_enhances_the_outgoing_opinion}</a><br>
<a>flag{the_reward_contributes_the_ray}</a><br>
<a style="display:none;">flag{the_user_checks_the_sourcecode}</a>
<a>flag{the_person_tutors_the_experience}</a><br>
<a>flag{the_water_revamps_the_spotted_food}</a><br>
That is the true flag, namely the_user_checks_the_sourcecode.