Red hot summer, red hot engine temp. Options

[tomazws]

I was driving on local street (grandpa chillin' style, shifts at around 2K~2.5K) when I noticed my temp gauge went rocket high. Immediately, I stopped the car on the side, popped the hood and let it chill for a bit. Then, I poured in about half a gallon of 50/50 (I have a gallon left over in the trunk from I forgot when), and tried starting the car. Everything seemed normal. I started driving the rest of the way to work and everything seemed normal...

Q's:

I poured the 50/50 into the tube until spill, it's about half a gallon I poured in. Does half a gallon of coolant really make an engine overheat?

Or

Would it be a faulty sensor?



This is the first time ever the temp gauge wasn't in the middle.
It should be about 86 ~ 90 degree outside.