well, isn't this the old problem of alignment in D&D?
basically I would agree with those posted before me, that a single good or evil act of great magnitude does not in itself make the actor evil or good by itself.
Let's take something wholly evil... a vampyr f.e. ... and he manages to kill a lich that planned (and nearly succeeded) in taking over the world... but the vampyr did it, because he hated the lich, not because he wanted to save the world... would this vampyr suddenly be good?! No!
Therefore the intentions behind the actions do matter.
But here comes another one - it is said, that in the D&D-cosmology alignment is not dependable upon personal opinions... like, the guy who poisons the well of a city is evil, even if he believes (for himself) that killing all those people is mercyfull toward them and therefore believes himself that his act is a good one (and therefore has good intentions)...
now, following this, I think only good intention doesn't make a good person.
but, as learned from the first one - good acts don't make a good person either...
this would lead to the assumption, that only if a person has good intentions and in his believes a good act is the same as in the general D&D-cosmology, then he is a good person.
last, but not least, there is the good cleric that goes to war with his army against another good army (which he believes are bad). Now, the good cleric naturally believes he is on the rigtheous side and therefore, by attacking the bad country, does a good deed (he is having good intentions). Furthermore (assume) his believes go conform with the D&D-Cosmology.
Last is the question - a good person with good intention (in terms of D&D-Cosmology) doing an evil act (would be killing a good cleric be an evil act? - never mind) - would he be considered good? Let's assume not (allthough I'm not 100% convinced).
this all would lead to:
A good person is a person that does good deeds with good intention that have to go conform with D&D-Cosmology.
Back to the original question. I do not believe, that if a "good" person did one "evil" deed without evil intention would cease to be good on the spot. If his intentions where good (which need to go conform with D&D-Cosmology) he would (IMHO) stay alignment "good". Allthough, once he sees the result of his actions he might feel an enormous amount of guilt - which can easily shake his believe in the good side and make him a possible suspect to corruption.
Now, if the intentions where good, but his believes does not go same as D&D-Cosmology, we are having the same situation as the example above (the one where someone poisons the city well). In this case, yes, I think the character should switch to evil immediatly.
Last, if his intentions where selfish or he had other evil intentions, again, yes, he should certainly switch to evil.
This all does not mean, that a god or other divine power wouldn't most certainly disapprove of the act and might very likely take away a paladins or clerics powers until attonement.
In the first case, I would think, that redemption would be possible, in the later two I do not believe (allthough you should never say no).
The whole problem with all of this is the general "alignment"-thingy of D&D-Cosmology.
Alignment there is a general overall thing and not a personal thing.
This makes it very difficult, once a characters believes of alignment does not go conform with the D&D-Cosmology. Let's take for example the assassine that uses poison to try to kill off the enemy evil archmages, because otherwise these would overrun, kill, enslave his own country. This assassine cannot be good or even neutral by D&D-Cosmology-standards. Allthough, the assassine might think of himself of a very heroic and good person (rescuing a whole country might lead to this assumption). And this is the whole conflict - I think it would be better to have good & evil be defined by the person personal believes (and possible influenced by a church or divine power if the person believes in this).
just my 2 cents
Azal