@wpsdlrg: Which gun did you have and which part broke? There is no reason to defend a manufacturer/gun on the internet. People might want this information so they know who/what to avoid.
Yes, when guns break, bad things can happen (but this applies to every gun). Do you carry revolvers? Because, even though you can carry a Sig, HK, etc with the hammer down, that won't help you with this failure. The hammer is cocked after the first shot. So if the equivalent event occurred with a Sig, HK, etc those guns would go full-auto also (you were firing the striker-fired pistol when that part broke so the Sig, HK, etc would have been cocked as well). Also, the firing pin block never had a chance to work in your situation because you had the trigger pressed all the way to the rear (which deactivates the firing pin block).
If the issue is that this failure can even occur in your holster with a cocked PPQ. I assert that this situation still couldn't happen. Since you haven't fired the gun yet and you aren't holding the trigger to the rear the firing pin block is blocking the path of the striker (it hasn't been pushed up and then gotten "stuck" yet).
Its really easy to clean the firing pin block, too. On the PPQ you simply have to apply slight external pressure on the extractor, then fully depress the plunger located aft (rear) of the firing pin block. This will allow you to remove the extractor from the slide. Then, you can simply pull the firing pin block and spring out of the slide. You are then free to clean the firing pin block and firing pin block channel to your heart's content. Make sure you keep the firing pin block channel and striker channel debris free and dry (or VERY VERY minimally lubed) and you are good to go.
I mean no offense. But, I don't like misinformation. And, just to be clear, I'm not labeling what you said misinformation, yet. I just see flaws in your reasoning and I wanted to give you the opportunity to address them. I don't want the OP to be misinformed.
Rifle: Tavor, Custom AR-15
Pistols: Colt Python, XD(M) 4.5" .40, Glock 19, M&P 9mm, Walther PPQ First Edition 9mm, Sig Sauer P229 Scorpion 9mm, Walther PPS 9mm, Browning Buck Mark