WhenAs like 80% of the sharepoing developers, if you are deploying your solutions trough command line.
WhatAfter firing deploysolution, looking at system settings > farm solutions and the error looks something like "Can not overrite xxxx file, access is denied".
SolutionIt was due to my source file was read only. I tried by un-checking the read only attribute from properties for source file and tried the command again. I got the same error couple of times and repeating the same solution fixed the problem.
ConclusionAs a social animal, when I get such kind of error first time in my life, I first ask to couple of developers around me if they have seen the same error ever, that helps a lot. When I do same, I got many useful hints that can either quickly solves the problem or useful for some future problems.
Update - 3-Jan-2011:If you are having more than one application server in farm then you have to go through all of them and uncheck read only to get it working.
Got the same error? Don't worry try above solution and let me know if same OR some other solution worked for you.
Happy sharepoing deployment!