Recently I have implemented transparency (all modes I think) in my render engine. It seems some tools exported transparency parameter wrong in past. They may have fixed that bug now, but there are many exported files with this bug.
I applied a bugfix in AssetKit for files which exported by SketchUp with under 7.1 version. It simply changes transparency like this: transparency = 1.0 - transparency. I did this because Apple also fixes that bug for SketchUp files and I appreciate that. Now by simple bugfix I can render old and new SketchUp files.
I wanted to share this to encourage other tools to do that, also one other reason is that I want to know other bugs which I can fix while importing. I don't think I want to fix all bugs 
Also I think we can create a wiki page to let other communities to know about which exporter has which bug. It also would be great to see that exporters' authors/companies will update that bug page by time. Maybe there is a bug page (exporter bugs not COLLADA's) which I don't now, I don't now.
Bugs I fixed:
1. If authoring_tool contains SketchUp and version <= 7.1 then
transparency = 1.0 - transparency