I notice that on my new 5-1 system the 12 to 24 hour forecast shows rain even though it's clear, 85 degrees and no rain is in the forecast. Is this rain forecast simply a "mistake" because the units new and is still in the Learning Mode?
Maybe, but one thing you need to remember is that it is only basing a forecast on very limited data from one sensor and some basic forecasting rules for your geographic region (limited to North America). It doesn't have access to things like radar, satellite, and live professional meteorologists running computer models and simulations.
So you have to take the forecast with a grain of salt. I find mine are typically in the general ballpark, but I don't go any further than that.