1. Does SOH CAH TOA apply to non-right angled triangles?

No, the triangle MUST be right-angled. Non-right-angled triangles have special formulas called the SINE RULE and the COSINE RULE.

2. What is one of the most common mistakes made by people using a calculator in trigonometry problems?

They have the calculator set on RAD or GRAD instead of DEG (degrees). 
Use the button marked DRG to ensure the calculator is set on DEG.