Plugin, WITHOUT White Space, Creates Header Error. Baffled

Hey,

I know that whitespace/etc can cause header errors. But my code
simply doesn’t have any. The error is listed when I try to make a new post:

Cannot modify header information – headers already sent by (output started at /home/wwwulti/public_html/blog/wp-content/plugins/catbocker/catblocker.php:1 in /home/wwwulti/public_html/blog/wp-includes/classes.php on line 806

I know there is no trailing space/whitespace. What other factors could cause this and how can one prevent it? I swear, I am doing my best to search for an answer but I cannot afford to lose a day of production on this. Hopefully somebody else experienced this too.

TY for your time.