Out of the reports we are receiving from our users, we have selected some of the user errors which seem to be more frequent than others. Below is a list of the most common ones:

Wrong Albedo (material brightness)

Wrong material Albedo, especially on large flat surfaces, may be the cause of very slow rendering or unrealistic look of the rendered scene.

See: What is Albedo?

Using a big sphere or cylinder mesh surrounding the scene as environment lighting

Such a scene setup, apart from being unpractical, will greatly increase rendering time.

The correct way to create environment lighting is by using environment settings 

See: How to set up HDRI environment?

Misuse of Corona Light material and Corona Material with self-illumination

Wrong use of these two kinds of light-emitting materials may lead to excessive noise and slow rendering.

See: Corona Light material or Self Illumination?

Placing lights behind windows

Artificially adding brightness to the interiors should be avoided as it may introduce noise, long render times, and unrealistic results. It is recommended to use environmental lighting and optionally portals (the use of portals is recommended just in case Corona renderer 5 or below is being used), which may greatly reduce render times.

See: How to set up HDRI environment?

Using the wrong glass type

Generally, thick glass should be used for solid objects (liquids, vases) and thin glass should be used for flat surfaces (glass panels) and objects with very thin surfaces (soap bubbles).

See: What type of glass should I use?

Using refraction for leaves or grass materials

Never use refraction for leaves (and all other kinds of vegetation and geometry with no thickness). Use translucency instead.

Always using Ambient Occlusion to enhance shading
Unless intended as an "artistic" effect, Ambient Occlusion should not be used for shading. For realistic purposes, it should be used to simulate "dirt" or to add different colors in concave/convex areas of the object.
See: How to add dirt effect to objects?

Using texture maps in Fresnel IOR instead of Reflection Color slot

To vary the reflectivity of a material, you should usually place a texture map in its Reflection > Color slot. When placed in the IOR slot, texture values lower than 1 will be interpreted as 1/value. This allows using low dynamic range images as IOR textures without output manipulation. This has been introduced to improve compatibility with applications such as Substance Painter.

Using features of other renderers with Corona

Using features (plugins, options) available for other renderers is risky, and is never guaranteed to work.

Example: using V-Ray HDRI loader as an environment map may lead to flickering when rendering animations with UHD Cache.

Wrong render settings

Default render settings should work well in most situations. Sampling settings should be changed only when necessary and when the user understands what he is doing.