To temporary substitute your superior (when absent or sick or both) may seem the ultimate chance for careerbuilding. Finally taking the opportunity to do the things your superior was unable to is also a huge ego-booster. The catch is in the fact that it's a temporary situation: on his return your superior will take credit for all your good work and, because you did all the work for him, he will have all the time in the world to think of a plan to get rid of you. Never ever substitute unless you're absolutely sure it's a permanent situation (in wich case it's called a promotion).

(Jacco Mokveld)