Here’s a serious mistake that could be very dangerous.
This is a picture of a gas furnace with an air conditioning evaporator coil above it. The gas pipe is made from black steel pipe and begins at the left. After running through a red-handled valve that is, thankfully, turned off, the gas pipe runs into the condensate drain opening for the air conditioner coil. If you were to turn on that valve, gas would flow into the duct that contains the AC coil. Given enough time, the gas would travel through the ducts, flooding the house with gas and creating a potential for a house-destroying explosion.
In a proper installation, the black pipe would have run into the gray control valve near the bottom of the picture.
Aside from the obvious error in connecting the gas line to the wrong opening, it’s clear that whoever did the work was not an experienced installer. No professional would have smeared so much blue pipe sealant all over the place.