Over lap

Lack of fusion

Surface porosity

Small cracks in bead

Over heating

Improper flame

Inadequate preheating

Improper cleaning and less use of flux

Insufficient root gap

Welding current too high

Wrong angle of electrode

Use of a very short arc length

Porosity

Under cut

Lack of fusion

Lack of penetration

Cause to over lap

Cause to back fire

Cause to porosity

Cause to flame snape out