In SharePoint 2010, Mobile Adapters were introduced to create the mobile device functionality for custom webparts when needed. Along with being quite laborious (as anyone who has created a webpart from scratch in previous versions knows), there are ‘pointers’ which are entered into the Compat.Browser file which tell SharePoint, if the device viewing the page is a mobile device, instead of showing the regular wepart on the SharePoint 2010 page, use the mobile adapter instead.

As it turns out, when updates are made to SharePoint (from Microsoft), this Compat.Browser file (inside App_Browsers folder in the root of the site) is subject to being overwritten. When that happens, the mobile adapter entries you’ve created get wiped out. And when THAT happens, the end user just sees a blank page.

I strongly suggest that you save your mobile adapter entries in some form (SourceSafe/TFS/Whatever) so that when and if this happens, all you need to do is to copy from your backup file and paste them into the newly created/overwritten Compat.Browser file.