I have been getting the following error on a new site that has several hundred pages of content:
Cannot modify header information - headers already sent by (output started at /hom/edited/edited/includes/bootstrap.inc:557) in /home/edited/edited/includes/common.inc on line 142.
I have been scouring these forums and even googled the problem. The broad consensus is that it is almost invariably a "whitespace" error. However, I have been through all of my pertinent include, module, and theme files and they all look fine. I have even looked at some of them with a hex editor to make sure I wasn't running into the dreaded BOM error.
There are a few things worth noting.
-I am using Drupal 4.7.6.
-First of all, this problem only occurs with anonymous users and cache enabled. It never occurs with cache disabled.
-It seems to occur completely at random although with some frequency. (This site is getting 2k+ page views a day, and an error will pop up say every five minutes or even more. I can reproduce it by visiting from my computer as an anonymous user although I may have to visit a dozen or so pages to get it to happen.)
-It seems to happen more on pages that have a lot of data, but it seems that it can happen on any page (most of my pages are text files).
-The error message always references the same two lines in the same two files (see error message above)