Sorry, don't own a camera so will have to mostly list things off.
Parents' home, ~3600sf, filters changed regularly, clean unit (20 years old, though), but cold/hot in the kitchen (depending on the season). Was asked to house-sit while they were away, and went to check on their HVAC. Here's what I found:
High-amperage (>20) breaker feeds furnace (100k GAS, w/ 25A fuse), foyer receptacles (WTF?), and I can't remember what else.
Duct never connected to register in den.
No balancing dampers anywhere (helpful for the 5' run to the nearest versus 50' to the furthest).
With above, badly designed ducts (no use of Manual D).
Vent too close to elbow, recieves no air when fan runs.
Return in dining room never connected, 12x12 ends (open) in furnace room and grille goes nowhere from dining room.
Another return duct (joist panning) never capped at end of panning.
Almost no airflow to kitchen, forcing use of space heater.
I work maintenance, not HVAC, but I knew I had to do something. Until I can get someone with more experience, and a license, in here I made a few changes.
Using "available materials" (interpret at will) I capped both open return ducts (and sealed a supply duct I found blowing air at me in the process), shut down the 5' supply vent, capped the useless vent in the elbow, tweaked the airflow from another vent, and installed a new programmable t-stat.
After all was said and done my parents will no longer need to use a space heater in the kitchen. Funny what happens when you finish your work.
Now I just need to get someone more qualified in here to make permanent repairs.