Let a defect, which is possibly but small, appear undisguised. A fault concealed is presumed to be great.

[Simpliciter pateat vitium fortasse pusillum:
Quod tegitur, magnum creditur esse malum]

Martial (AD c.39-c.103) Spanish Roman poet, satirist, epigrammatist [Marcus Valerius Martialis]
Epigrams [Epigrammata], Book 3, Epigram 42 [tr. Bohn (1871)]
    (Source)

Alt. trans.:

• "Conceal a flaw, and the world will imagine the worst."
• "Simple decays men easily pass by, // But, hid, suspect some great deformity" [tr. Anon. (1695)]
• "Double we see those faults which art would mend, // Plain downright ugliness would less offend." [tr. Sedley]
• "Let a blemish, which perhaps is small, simply show. The flow which is hidden is deemed greater than it is." [tr. Ker (1919)]