The error means that the script handling whatever you're trying to do (post a usernote in this case) is eating up more memory than PHP is allowed.

It's reproducible currently by attempting to view the announcements from within the Daily Grind forum which is extremely strange (viewing an announcement shouldn't eat 32MB of memory), so it's possible that some other process is pushing PHP past the limit.

It could be caused by a memory leak or just by having too low a limit on memory. Searching for short strings consumes a lot, so perhaps that was happening when the error occurred.