Unintentional textured pattern left in the paint by the roller.
Use of incorrect roller cover.
Use of lower grades of paint.
Use of low quality roller.
Use of incorrect rolling technique.
Use the proper roller cover; avoid too long a nap for the paint and the substrate. Use quality roller to ensure adequate film thickness and uniformity. High quality paints tend to roll on more evenly due to their higher solids content and leveling properties. Pre-dampen roller covers used with latex paint; shake out excess water. Don't let paint build up at roller ends. Begin rolling at a corner near the ceiling and work down the wall in three-foot square sections. Spread the paint in a zigzag "M" or "W" pattern, beginning with an upward stroke to minimize spatter; then, without lifting the roller from the surface, fill in the zigzag pattern with even, parallel strokes.