My neighbor was having issues with her car not starting (08 Mercury Milan). It would "click" when you tried to turn it on. After 3-4 tries it would spark to life. Her mechanic said it was definitely the starter ($200 parts / $150 labor). I took a look under the hood to test her battery along with the cables. The battery was still solid and the cables were snug on the terminals, but I noticed a tiny bit of acid/corrosion around the connectors/terminals. I cleaned it off and sprayed some protective cleaner on the posts/connectors. The car now starts without issue.

I wanted to post this to say thanks for always reminding people to start with the battery first when there are electrical issues. Saved my neighbor $350 and it took me 5-10 minutes to fix.