Tanks would be peppered with artillery whenever possible. The odds of an artillery shell or mortar shell hitting atop one doesn't seem that remote to me. Consider these unlikey but supposedly true WW2 anecdotes:
(by B Cooper, "Death Traps")
1) An M4 Sherman was disabled when a high velocity round (probably AT) came right through the length of the 75mm Sherman gun tube. The breech happened to be open and the round exploded, killing all three turret crew.
2) An American heavy bomber at about 10,000 feet (B17 or B24) exploded overhead when hit by German flak. Its tail section spun earthward, crashing near Cooper's unit. Inside was the unconcious but uninjured tail gunner.
(by D Loza, "Commanding the Red Army's Shermans").
3) During a Jabo stafing attack, a German 20mm shell entered through another Sheman 75mm gun tube. The shrapnel of the 20 mm shell killed an NCO inside the Sherman's turret
4) A suicide kamikaze attack on a Soviet armored column during the last days of WW2. The lead kamikaze fighter dove right into the glacis of the lead Sherman! Inside the "emcha", a stunned but uninjured Russian driver. The plane was obliterated -- the Sherman was still functional! Two other fighters dove into a building where Russians had sought shelter, three others were shot down by Russian AA fire. No Russian casualties besides the shaken Russian driver. Loza said that several of the Japanese planes had a woman along with male pilots. Loza conjectured that they were girlfriends/wives of the desparate Japanese kamikaze pilots.
5) <<this is the most amazing, true or not?>>> Ju88 medium bombers were circling overhead where a group of Soviet M4s were seeking cover under some trees. Knowing that when a tank emerged, the Ju88s' tactic would be to dive onto the lone target, one Soviet tank commander charged out of the tree cover towards the upward slope of a hill. Once on the slope he raised his gun to maximum elevation, gave the order to fire an HE round. It obliterated the Ju88 diving towards it. The rest of the 88s scattered and left the scene.
Very interesting reading, to say the least!