Although the symptoms vary, one of the main culprits is a mismatch between the version of Project Pro on users’ machines and Project Server. For example, if you deploy SP2 on the server, you must ensure that you also deploy the corresponding patch to ALL machines with Project Professional that are used to connect to the PWA site. Unfortunately, there appear to be protocol and/or behaviour changes across server + client patches (hotfixes, Infrastructure Updates, and Service Packs). If the client and server do not match, these differences can be close enough to still pass initial validation, yet different enough to lead to subtle problems in your project. I’ve seen data loss in notes columns, as well as total corruption of projects requiring rollbacks from backups, desktops, or the published database. Probably the scariest part of this problem is that you need to ensure version standardization of Project Pro across all of your users – all it takes is 1 bad apple to introduce corruption or lose others’ data. This version mismatch often results from fairly innocuous sources, such as a fresh install for a new user (resulting in deployment of RTM), or a user with admin rights applying their own service pack (I saw a lot of people go to SP2 this way), or an IT dept applying a patch without coordination with the server team. Also, take note that this greatly complicates the procedures necessary for testing + deploying patches to your Project Server environment, as you need to ensure simultaneous deployment of the server + all client patches. This is pretty much impossible without System Center or similar tools. Inevitably, there will be stragglers due to PCs that are turned off or otherwise fail to patch. Schedule time to isolate unpatched machines + to perform cleanup before they can connect to Project Server. Good end-user communication works wonders here to (I suggest issuing a “please check your version by clicking Help->About Project and verifying you see the following numbers before starting work on Monday” message).