Oak Wood at 0/0/0 instead of Bedrock

Oak Wood Block appearing at X=0 Y=0 Z=0 instead of Bedrock

What I expected to happen was...:
Bedrock should be in place like everywhere else

What actually happened was...:
Oak Wood Block replaced Bedrock

Steps to Reproduce:
1. Go to coordinates X=0 Z=0
2. Dig down to Y=0
3. Observe Oak Wood