I (and many others on these forums over the last few years) get random compile errors with 'core.a' about every four or five compiles. I just hit the upload button again and it usually goes through OK the next time. Sometimes I have to try a few times before it compiles properly. Sometimes I can't compile until after I take a break and have had a cup of coffee.
I have IDE 1.0.5 running on XP32 loading to Unos and Nanos. Before you ask - Yes I do have the serial port and the board type set correctly. This error has been a pain in the a.. for a long time and it seems nobody can fix it. I have tried just about everything I can think of except buying a new laptop but I haven't found anything that eliminates this PITA Arduino IDE bug.
I also sometimes get double sketches opening at the same time. I have to constantly keep checking how many sketches are open and close any duplicates that open. Maybe there is a connection? The Arduino IDE is a bit flakey but its free so I live with it since I have no choice.