Memory problem...

I've posted this before, but still haven't been able to solve it. Often, users will get "Fatal error: Allowed memory size of 15728640 bytes exhausted (tried to allocate 86460 bytes) in Unknown on line 0". I'm not doing anything very memory-intensive, and this sometimes does mention a specific line, which is often fairly early in the file. Using PHP 4.1.1 with the ioncube optimized. This also happens to other sites on my server. Is there some sort of memory leak here, or...? -- The above message is encrypted with double rot13 encoding. Any unauthorized attempt to decrypt it will be prosecuted to the full extent of the law.