If you have a development store e.g. mainstore,com/dev and you forget to update the web address in storefront settings e.g. it remains mainstore.com instead of mainstore.com/dev can that corrupt your main store or does that simply mean it will display mainstore.com but all changes you make will still occur in mainstore.com/dev (scenario assumes you have made the appropriate change in local_config.php to path and DB)
No, it cannot corrupt the main store because it uses its own database (or at least it should use a separate database if everything is done correctly 
). And yes, it will simply show the storefront of the live installation instead of the dev one.