Multiple SQL Server data files on same SAN disk
Even multiple tempdb files on a single LUN should be a last resort compared to one tempdb per LUN.
If there are multiple database files on a LUN, and they are allowed to auto-grow, their contents will be interleaved in the filesystem and on the storage.  Hit them both heavily with read or write requests, and a situation known as IO weaving, or disk head thrashing will result.  Even though writes are generally cached, writes are not immune to io weaving, because eventually the writes need to destage from cache.
If tempdb is used heavily by query spill, there will be a lot of sequential write activity.  Put multiple tempdb on the same LUN, and there can be multiple streams of sequential write activity at different locations on the storage, all accessed through the same LUN queue.  Sequential reads such as a table scan introduced on top can encounter worse-than-random read latencies.