Broken printing in an enterprise Java app - it was relatively serious as customer's business process was relying on collaborating using prints. There was no message, no error & it was only failing for one specific customer.

It took us 2 weeks to found the problem. From what I remember some entry in the middle of windows PATH variable had a problem. While loading DLLs this caused a silent failure, and caused printing DLLs not to be loaded. It was all in Citrix environment which definitely did not help.