I've tried converting phpbb3 to smf (latest versions for both). That worked fine with no errors. Next I tried looking around the newly converted forum and I would get blank pages. Not all the time, just most of the time. error_log is blisfully empty so there aren't any obvious things wrong.
Spamming refreshes solves it sooner or later but I can't expect my users to do that.
Everything else, posts forums etc seems in working order.