TOP (n) without ORDER BY can return different results between executions of the same query. TOP 1 effectively means "give me a row from the set matching the search criteria, any row will do". That's very different to TOP 1 ... ORDER BY, where the first row of the ordered list would be returned.
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Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden