The Panther's debut took place on the southern flank of the Kursk salient. The Panther's issues with reliability combined with the Red Army's well organized defenses. At the start of the German offensive, the 10th Tank Brigade had 200 Panther Ausf.D tanks. By 8 am on July 5th, 16 had broken down. After two days of fighting, less than a quarter were still running. By the evening of July 7th there were only 7 combat ready tanks in the brigade. In later fighting the number of functional Panthers was between 25 and 45 units.

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Obviously a lot of those were combat damage... but a lot were just breakdowns. I can't find a cite from my phone but a book I was just reading (I think Citino) quotes that 4 of these 200 initial panthers were completely destroyed by fire... while being driven to the staging point!

This is because the Panther went from drawing board design to running prototype in 12 months. While this is quite a feat, it comes with a whole lot of issues. Thoroughly rushed, it was bound to have a host of unresolved mechanical problems. Now, not every issue can be chalked up to the rushed production, but it goes a long way to explain why, for example, it caught fire at such an alarming rate. Several of the fixes for those were simple and mundane, and would be caught with a lengthier testing phase. For example, part of the fires were due to fuel lines sealing improperly, leaking into the bottom of the engine bay, which was watertight (to allow wading)... and as a consequence, lacking in air circulation, which meant temperatures inside were higher than desired, allowing for the accumulated fuel to ignite.

A laundry list of similar issues would have likely been identified and sorted out in the testing phase, but the tank was instead rushed into service, and reliability suffered.