What precautions should I use when writing assembly code & using sprites ?

If your writing GBC dedicated code then the following info doesn't apply.

If you are writing GB or GBC compatible assembly code then you need to make sure that the following instructions don't occur while their double register values are in the range $fe00-$feff or else sprite RAM will once in a while get trashed causing sprite flicker & other strange sprite behaviour:

inc xx
dec xx

(xx = bc, de, hl, or sp)