most problems in welding are caused my the welding puddle being contaminated in some way. Lack of shielding gas (empty tank, of wind blowing it away) lets oxygen into puddle. Dirty material (rust, paint oil) allow the contaminants into the puddle, which in turn can cause the weld to distort or crack, or even burn through the base material.
Inconsistent torch angle while welding causes variation within each quench line (appearance of weld,) which may cause a weld to be undersized based on a fabrication print or could cause undercut by not filling in an edge that was melted away.
Clean and consistent welds, with the proper prep routine will usually work in the end