You mean some of the retired GM workers lost their pensions which was the one of the main driver of the GM's bankruptcy? It's prime example of how unions bankrupts its own company.
Also, most GM workers didn't even lose their pensions. Obama made one non-negotiable condition for GM's bailout and that was keeping the pension. So, they removed most of other debts and the new GM kept the pension.
Ironically, massive bailout and pension obligation caused pension insurance premium to shoot up, causing other companies unable to afford to offer pension anymore. GM union workers are the reason we don't get pension anymore. They screwed nuts and bolts and wanted 6 figure salary with pension. Of course GM couldn't compete and bankrupt.