I found a very interesting website about mapping errors when I was looking for possible fixes for a recent problem I have when compiling, and I thought to share it here:

http://www.cstrike-planet.com/tutorial/24-Exceeding-Limits/5

It looks like the poster for all these useful tutos and posts was Satchmo

For generic lamps and lights:

Use a light_spot instead of a light entity whenever possible. It casts much more realistic looking light because it is directional, even if you want to make the radius angle large.

For lamps and some other light types, use point_spotlights. When compiling your map in HDR they look great and massively boost the awesomeness levels of otherwise boring bright lights.

Use contrasting colors or flickering lights to signify something important. The player may not consciously realize it at first, but lights are a great guide as to how to navigate an area.

It's never a good idea to have extremely brightly lit areas or pitch black areas.

For projected textures, put them behind objects or grating. The high quality textures and texture shadowing with bump maps really help realism and interest.

I found this to work recently... if you map with a fellow mapper over Skype (or next to you), and you work together, you can create stuff exponentially faster and higher quality.

Extremely amazing trick to enhance the normal maps!

Here is a useful program used to Decompile .MDL Into .SMD So you can put your Models into blender or other 3D Modeling Software. The program itself is pretty easy to use, It is called Crowbar. And it ACTUALLY Works! Unlike many .MDL Decompilers it Crowbar WORKS!

Crowbar: http://bit.ly/CrowbarDecompiler

Valve Developer Community Page: http://bit.ly/CrowbarValvePage

Something I discovered recently that could help anyone not too used to working with P2's instances:
When adding light panels, never use the ones that end with "detail" unless you're putting them in extremely tight places.
The normal light panels have nodraw-textured backs added to them, to help prevent leaking. Meaning if you're fitting a light into a wall that's 8 units thick or more, the light should be completely sealed without needing borders, except maybe on the top and bottom in some cases.
The light instances that end with "detail" don't have these backs on them, because they're meant to be shoved into tight spaces, like a wall with a room on both sides, for instance.

I just wanted to take a quick moment to highlight the importance of bloom and color correction to create moods

Normal: http://steamcommunity.com/sharedfiles/f ... =419244130
With bloom and color correction:

http://steamcommunity.com/sharedfiles/f ... =419244267
With Color Correction:

http://steamcommunity.com/sharedfiles/f ... =419244387

I found an interesting little feature when trawling though the 2013 SDK code. The ParentName keyvalue actually can also contain an attachment name. Setting the parent to 'model,panel_attach' will parent the entity to 'model', and then SetParentAttachmentMaintainOffset with the 'panel_attach' attachment point. Hammer doesn't parse this, but it works ingame.