When copy data from MS Excel sheet to SharePoint list datasheet view, some items triggered workflow successfully, but some got "Failed to Start" error. For the failed ones, if I delete them and try again, normally the errors disappeared.
There are one application server and two web front servers in the SharePoint farm. NLB (network load balancer) is created for the two web front end servers. The "Microsoft SharePoint Foundation Workflow Timer Service" service is running on the application server.
Let's keep this post short. I started the workflow timer service on all servers, then the problem is resolved.
I think there is a bug with SharePoint timer service (SharePoint 2010 + sp1 + CU201206), and it cannot handle NLB requests properly.