Due to the easy nature of the DJI Spark, the equipment that can fail is quite small.
Battery
Do NOT fly if the battery appears dead or a problem is suspected. Low battery signs should NOT be ignored and the drone should be landed ASAP
Phone
The DJI link uses WiFi from a mobile device to control the drone. If the WiFi link or phone appears to be unreliable or not responsive to commands, the drone should not be flown.