It is possible that trazodone can rarely cause a false positive for LSD. However, when the urine is sent for confirmatory tests, it will be shown to be a false positive.
There is a web page that summarizes the reports in the medical literature at http://www.askdocweb.com/falsepositives.html.
The only reports with trazodone are isolated reports, so is rare and an exact percentage cannot be calculated. The ibuprofen is a more common cause of false positives, varying from 1-5% in various studies. Again, if a false positive occurs, performing a confirmatory test will clarify that it is a false positive.