Little Hagia Sophia is a small church in Istanbul located close to the famous and large Hagia Sophia. It was built around the same time and is said to have contained very beautiful mosaics.
At the time of Turkish conquest of Constantinople, Hagia Sophia was converted to a mosque and the mosaics were plastered over due to the prohibition of religious icons in Islam. Similar fate befell Little Hagia Sophia about 50 years later. Fast forward to today, the church's dome and interior is still plastered. I could not find anywhere whether the mosaics underneath are still preserved waiting to be uncovered, or have they been removed entirely?
(Interestingly, there is a church in Ravenna with the same layout and possibly the same mosaics, also built during the reign of Justinian).
Wikipedia article: http://en.wikipedia.org/wiki/Little_Hagia_Sophia