After a TNT is ignited,it will flash several times as intended.Then,when it is supposed to explode it will disappear.At this time,any blocks placed at the original TNT position will disappear.After a while it will explode.If you exit and enter the map again,the invisible TNTs will show up and explode as normal.This bug usually happens in amplified maps but can happen anywhere.

pic.1:TNT placement
pic.2:primed TNT
pic.3:disappeared TNT
pic.4:exploded TNT