From GBATEK, writes to a certain I/O port for power management may damage hardware:
4000304h - NDS9 - POWCNT1 - Graphics Power Control Register (R/W) 0 Enable Flag for both LCDs (0=Disable) (Prohibited, see notes) 1 2D Graphics Engine A (0=Disable) (Ports 008h-05Fh, Pal 5000000h) 2 3D Rendering Engine (0=Disable) (Ports 320h-3FFh) 3 3D Geometry Engine (0=Disable) (Ports 400h-6FFh) 4-8 Not used 9 2D Graphics Engine B (0=Disable) (Ports 1008h-105Fh, Pal 5000400h) 10-14 Not used 15 Display Swap (0=Send Display A to Lower Screen, 1=To Upper Screen) 16-31 Not used Use SwapBuffers command once after enabling Rendering/Geometry Engine. Improper use of Bit0 may damage the hardware?
Emphasis mine. The document does not go into detail and provides no concrete information. How could improper use of the enable flag for LCDs cause hardware damage? Under what circumstances would it cause damage, and what types of damage to the hardware would it cause?
As pointed out in a comment, this may be similar to a related issue on the Game Boy:
Stopping LCD operation (Bit 7 from 1 to 0) may be performed during V-Blank ONLY, disabeling the display outside of the V-Blank period may damage the hardware. This appears to be a serious issue, Nintendo is reported to reject any games that do not follow this rule.
I've posted this after writing a meta question about the NDS and receiving the answer that it may finally count as retro.