If you’re doing a multi-temperature mash by adding heat to your mash tun, you need to be wary of heat plumes fooling your thermometer. I have an 8 gallon pot with a false bottom that I use for my mash tun. When I do a multi-temperature mash, I monitor the temperature of the grain/water mash as I’m adding heat. Occasionally I’ve been thrown into a panic as my temperature seemingly shot past my target temperature.
However, if you’re not running some sort of recirculation pump you need to be wise to these tricks and give your mash a stir to ensure the hot water at the bottom gets mixed in to the entire mash instead of just finding an escape route to the top past your thermometer probe.
Similarly, when first mashing in your grain to your hot water, you need to give it about 5-10 minutes to totally equalize and balance out the temperature between grains and water before trying to adjust temperature up or down. I’ve had situations where it was location of the thermometer probe giving me an inaccurate temperature, or sometimes just simply giving things a chance to fully mix and stabilize.