On March 31st, 2011, my former boss left for lunch and didn't return. As he occasionally took afternoons off, this didn't raise any red flags with anybody. The next morning, we got an e-mail from him in which he stated he had retired. Because it was April 1st, and because of how he phrased his e-mail, it took a while for it to sink in that his e-mail wasn't an April Fools joke and that he had indeed retired.
In 2002, my entire dept. was being laid off. Though we worked until August, we were made aware of the layoff on 4/2. The HR dept. was writing up all the layoff paperwork on 4/1 and actually deferred telling us by a day due to April Fools.