When you start gathering and switch any gathering tool before the process is completed, the newly equipped tool has it’s use counter decreased, no matter what tool-type it is.
Example: You start harvesting a plant by pressing F and quickly equip a different mining pick, then this mining pick will have it’s counter decremented and the harvesting sickle’s counter will stay untouched.
It is not possible to gather higher-tier materials with lower-tier gathering tools, but considering harvesting sickles have only 50 uses but cost the same as mining picks with 100 uses, this could be exploited to harvest plants at half the price. Hardly worth the trouble but still an exploitable bug, at least by bots i guess.