Rotated die errors are pretty common, so the presence of "keys" and "flats" are apparently ineffective at preventing them. Some rotated die errors are fixed and some are dynamic. There are at least four possible causes:
1. A die becomes loose and rotates in its recess. (dynamic)
2. A die is installed incorrectly, despite the presence of correctly situated guides. (fixed)
3. The guides are placed in the wrong spot, leading to incorrect installation. (fixed)
4. A die is correctly installed, becomes loose, rotates, and then is clamped down in the wrong position. (dynamic, followed by fixed)
My sense is that dynamic rotations are somewhat more common.
Error coin writer and researcher.