This sequence is complete (cannot be extended in any direction), however, two numbers don't fit. Which two?
15 68 53 8 66 6 73 5 57
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Converting to element symbols we get P, Er, I, O, Dy, C, Ta, B, La. This almost spells out Periodic Table, but the two errors are 66 and 57 (and unfortunately cannot be corrected because no element has symbol Di or Le (and another spacing won't help either)