DO NOT use frozen meat as it will not safely cook. Don't put frozen or partially thawed meat or poultry into a slow cooker; they take longer to get to 165°F, and can also cool everything in the slow cooker. That's like building a bacteria playground. 

Your slow cooker might take a while to reach 165°F, which is widely regarded as the temperature at which most illness-causing bacteria are killed.

If cooking roasts, one should cut it into chunks so it can cook more evenly instead of one big chunk.