Handling S3 events. Race condition?

Is there a possibility of race conditions with S3 events? Does the event fire before the object is fully PUT? We are getting occasional "Unexpected end of JSON input" on the files that we are processing when handling the event. But upon manual inspection the file is valid and is not broken. Just wondering if possibly the PUT event is fired before the file is fully stored, and when we try to read it back in the event handler, we get a partial file back.