We are going to be turning on State/Country picklists soon for Leads and Accounts.
There is currently a process for us in place that inserts new Leads using the SForce SOAP Api). Data that is currently entered through this process (by the very nature of the process) is not currently validated, and can end up with some weird values (for example, the following could all be entered in place of United States: USA, US of A, U.S.A., Untied States, United State of America, etc - this includes misspellings and weird variants).
When picklists are turned on, will data like this that will be entered for country or state cause issues/errors when we try to insert it through the API? If so, is there any alternative other than to catch these inconsistencies and correct them (per the approved list of Countries/States) before submitting them through the API?