This took me a while to figure out... My Teensy 4.0 worked fine (with a connected LCD) but when I soldered an Audio Shield (Rev D, of course) under the Teensy, it stopped working and got quite warm. I triple-checked the orientation -- okay. I eventually figured out that one Teensy capacitor (C29) rests on the Audio Shield's SD (metal) card frame when the Shield is beneath the Teensy and the two are pressed together -- see the picture. I desoldered the Teensy, lifted it about .02", re-soldered and the problem cleared. A bit of Capton tape between the two is now called for in production. The picture has a cheesy piece of Scotch tape (too blurry to make out.) This is less likely to be a problem with the Teensy 4.1 because there are no big capacitors near the SD card cage.