Surly fixed/fixed w/16t & 18t cogs, different makes. I tweaked the chain line on one side w/2mm of spacer behind the 16, 'cause its thread flange was not as deep as the 18. Seemingly OK until I really torqued it going up a hill yesterday, and with fewer threads engaged, the 16 spun :( .

So I am down to single-side, and I will no longer use spacers behind fixed cogs. Shopping for a "thicker" 16 or a matched pair, and will adjust chain-line up front, if need be.