One customer reported over 50 errors, triggered by different plugins and core files with message:
Cannot modify header information - headers already sent by (output started at /wp-content/plugins/simple-mail-address-encoder/simple-mail-address-encoder.php:1)
I believe your file is not stored correctly. Most likely you have UTF-8 with BOM.
The bugs where reported through WP Bug Tracker.
Hope this will help.