Both Roberts and Polk say visible bra straps are a major pet peeve. Polk suggests wearing a tank top with wider straps or wearing a strapless bra (so obvious, yet so few actually do it). Roberts has another idea:

“One thing I see all the time when vacationing is the dreaded nude bra strap or side part of the bra showing. Ladies, this is a huge no no. Just because it’s nude doesn’t mean we can’t see it. If there is any chance your strap or side part of your bra is going to show make sure it looks intentional. Leave those nude winter bras and all other ‘under the sweater’ bras at home and invest in some delicate lace numbers with intricate straps.”