It is an asp.net compilation error. We see them all the time here actually (well, by all the time I mean probably once every week or so). It happens a lot when you are changing things and uploading a lot of changes to the website. Every time you upload pages the asp.net runtime has to compile those pages when they are first accessed. This is a known issue within the .net developer community, and Microsoft was supposed to release a fix for it in the last service pack for .net 2.0 but whatever they fixed didn't help most people.

For some people you have to actually go into C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\ and delete all the files out to get it to recompile the website.

Microsoft knows of one cause for the issue, and has issued a hotfix for it (number 915782), but there is a second issue that they haven't been able to nail down (as of last August/September) I have no idea where they are now with it.

Some have suggested that adding batch="false" to their web.config has fixed the problem for them, but again other say it doesn't help.

<compilation batch="false" debug="false"/>

Again, this is a bug in the asp.net runtime when it is dynamically compiling the website and not something that we are able to work around. If you don't believe me go google for "Could not load file or assembly App_Web" and see just how many threads and posts you get from all over the place. :-)