This may be common knowledge but I wasn't able to find anything about it while searching the forum, so I thought I'd mention it.
I was testing my first ATTiny application, a thermostat to replace the broken one in an RV propane/electric refrigerator/freezer, and found an interesting bug. My application uses ADC0, ADC2, ADC3, and three digital I/O pins for communication and SSR control. The ATTiny only has 8 pins so I am using everything it's got. ADC0 shares pin 1 with /Reset and some other functions.
I know what my temperature normally runs so I was testing the accuracy of each channel by placing the thermistor under my tongue. While doing this with ADC0, i noticed that every time the temperature reached around 85 degrees F, the processor would restart. It took me a few minutes to realize that this was at the Vrst threshold and that unless external reset is disabled by programming bit 7 in the fuse high byte, it is active no matter what you are using the pin for.
So, if you are not using the /reset pin for external reset, remember to program the proper fuse bit or your application may not work as planned. Also, unless you have a high voltage programmer, don't program the fuse bit until you are through programming the flash because programming hfuse bit 7 disables normal flash programming.
This just happened to be the ADC channel I picked for external temperature so if I had used it for refrigerator internal temperature I would likely have never run into a problem.