German tank production was indeed in a bit of a bind towards the end of the war for a number of reasons. One was that the factories were being bombed, two was that the armour composition had to be changed to compensate for a lack of alloying metals, three was that as German tanks migrated to thicker and thicker armour plates it was getting harder and harder to weld and condition them properly.

Due to damage from bombing, armour plates had to be stored in non-heated environments while they awaited assembly, which caused warping and cracking. When welded into hulls, they could be out of flat by as much as 10 mm. This was remedied by hammering in shims and welding around them, which resulted in a very weak weld that often cracked while the hull was still at the factory. Cracked welds were welded over again and again until they looked good, but the underlying structural integrity was already compromised. When Zimmerit was used, it was difficult to inspect weld seams for cracks. British sources state that the weld seams on a Panther were so weak that they broke up under small arms fire, but it's far more likely that the cracks were already there when the tank left the factory.

Interestingly enough, the Germans were aware of this, they simply didn't care. Even though the general stereotype is that German tank production was aimed at producing the best tank no matter what the cost, the reality was the opposite. As Dr. Washamht, the head of quality control at DHHV told the British: "100% sound work was not necessary - output was the main requirement". The drive to get as many functional tanks onto the front lines as possible coupled with the inefficient production process led to many taking shortcuts in order to meet their quotas.

The composition of the armour itself changed as supply of strategic materials ran dry. Not that the German armour was fantastic to begin with (that's a whole different story), but chemists had to struggle to keep up with the reality of war. By late 1942 molybdenum had to be excluded from thin and medium armour, by mid-1944 nickel was all but gone, by the fall of 1944 so did vanadium. However, these changes were the ones happening on paper. In reality, German armour production did not conform to their own specifications. Samples taken from captured German tank hulls and turrets showed that two samples of the same thickness taken from the same part of two different tanks could have vastly different chemical compositions. The same disconnect between specifications and reality happened when it came to hardness. The quality of heat treatment was so variable that even two plates with similar chemical compositions could exhibit different properties. This explains the results of several trials including the one held at Isigny: one of three Panthers tested had a much greater resistance to Allied shells than its two cousins.

American trials revealed the same properties. A sample taken from a Panther upper glacis plate (classified as "83 mm thick armour" in Soviet trials, found to be particularly prone to brittle failure) revealed that the sample "exhibited extremely low toughness...making it susceptible to shattering under a shock type ballistic test" despite being "sound cross-rolled steel". These properties were corrected by re-tempering the sample and quenching it properly.

The question of how prevalent these problems were is hard to answer. Metallurgical testing is destructive: in order to test a hull, a piece has to be cut out. This means it's impossible to test 100% of tanks, or you would end up with no tanks at all. However, as American, British, and Soviet sources all note the same defects when discussing the quality of German armour, it would appear that the issues were typical of German tank production, rather than freak one-offs.

Sources:

BIOS final report No.614 Item No.18 Welding Design & Fabrication of German Tank Hulls & Turrets

Report A.T. No.232 Armour Branch Report on Comprehensive Firing Trials against German Panther Pz.Kw.V.

Watertown Arsenal Laboratory Experimental Report No.WAL. 710/715 Metallurgical Examination of a 3-1/4" Thick Armor Plate from a German PzKw V (Panther) Tank

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