A little leaven ferments the whole batch of dough. - Gal 5:9
In context, this was regarding teachings (specifically circumcision) that were not in line with what Christ taught. Here's the context:
For such freedom Christ set us free. Therefore, stand firm, and do not let yourselves be confined again in a yoke of slavery.
2 See! I, Paul, am telling you that if you become circumcised, Christ will be of no benefit to you. 3 Again I bear witness to every man who gets circumcised that he is under obligation to keep the whole Law. 4 You are separated from Christ, you who are trying to be declared righteous by means of law; you have fallen away from his undeserved kindness. 5 For our part, we are by spirit eagerly waiting for the hoped-for righteousness resulting from faith. 6 For in union with Christ Jesus, neither circumcision nor uncircumcision is of any value, but faith operating through love is.
7 You were running well. Who hindered you from continuing to obey the truth? 8 This sort of persuasion does not come from the One calling you. 9 A little leaven ferments the whole batch of dough. 10 I am confident that you who are in union with the Lord will not come to think otherwise; but the one who is causing you trouble, whoever he may be, will receive the judgment he deserves.
My point here is that the statement, "Jehovah's people as a whole can never be corrupted" is actually in error. A little leaven (a little bit of false teaching, a little bit of hypocrisy, a little bit of lording it over, etc.) CAN corrupt the entire batch.