i think he means something like this. say a participant passes pretests, locks his solution, then makes an Unsuccessful hacking attempt. later he gets Hacked by another participant. then his score will become negative.
but despite this the participant can still be allowed to make more (possibly unsuccessful) hacks?

personally i think it should be allowed, because it's possible that all this can happen to an honest participant, and he should be able to redeem himself by making Successful hacking attempts.

PS: once i locked a solution and later figured out it was wrong. after this i made an Unsuccessful hacking attempt. i was so scared that i would end the round with a negative score!! luckily, i made three Successful hacking attempts after that and saved my performance in the round! nobody Hacked me throughout the round, but as expected i failed the system tests.

PPS: don't try to search for this round in my contests page! :P u wont find it, because (fortunately or unfortunately) it was lost in the Codeforces black day!