Some people do have isolated words which they mispronouce, and I think it is polite to just ignore their mispronunciation - unless of course it is Geroge Bush who persisnatnly says "Nu-cu-lar" for "Nuclear", and given the seriousness of the issues he is discussing, and his propensity for war-mongering, he shoiuld be corrected every time he says it, which is far too often!
Far more irritating to me is how ever news reporter on any TV or radio station pronouces the word 'secretary' as 'sec-a-terry', which is patently incorrect - that should be corrected because it is lazy speech, not an impediment.