Step 1: Fix the meaning of each word.
An error is a small, one-time mistake anyone can make. A fault is a defect, a flaw that keeps showing up in a person or thing.
Step 2: Match each word to its blank.
The first blank needs a word close to "mistake" since it defines error. The second blank needs a word close to "defect" since it defines fault. Only option B gives "mistake" for the first blank and "a defect" for the second.
Step 3: Rule out the rest.
Option A swaps the two ideas around. Option C uses "slip" and "an inaccuracy", both weak stand-ins for a lasting flaw. Option D uses "blunder", too strong for error, and "to blame", which does not fit grammatically.
Final Answer:
Error means a mistake, fault means a defect, so option B completes the sentence correctly. \[ \boxed{\text{mistake - a defect}} \]