der Makel
blemish
A small defect, imperfection, or spot that affects the beauty or quality of something.
Der Apfel hatte einen kleinen braunen Makel, war aber ansonsten perfekt.
The apple had a small brown blemish, but was otherwise perfect.

blemish / flaw
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blemish
A small defect, imperfection, or spot that affects the beauty or quality of something.
Der Apfel hatte einen kleinen braunen Makel, war aber ansonsten perfekt.
The apple had a small brown blemish, but was otherwise perfect.

The word 'Makel' (blemish/flaw) belongs more to elevated or written language. It often sounds more formal and is used to describe a defect or imperfection in a neutral way.
Die Vase war bis auf einen winzigen Makel am Rand perfekt.
The vase was perfect except for a tiny blemish on the rim.
Whether a physical or a character flaw is meant can be recognized from the context. If 'Makel' (blemish/flaw) refers to a thing (e.g., fruit, fabric, paint), the physical meaning is meant. If it refers to a person or something abstract (e.g., reputation, résumé), the figurative meaning is meant.
Der Tisch hatte einen kleinen Makel in der Holzoberfläche.
The table had a small blemish in the wooden surface.
In the figurative meaning, 'Makel' (flaw/stain) is often combined with the verb 'anhaften' (to cling to, to be attached to). You then say that a flaw clings to someone.
Ihm haftete seit der Affäre ein Makel an.
A stain had clung to him since the affair.
mistake / error
'der Fehler' and 'der Makel' can both mean something negative about an object or a person. 'Fehler' is more neutral and much broader: a device can have a 'Fehler', a person can make a 'Fehler' or have a weakness. 'Makel' is used more for an imperfection that affects quality, beauty, or reputation. The word often sounds somewhat elevated and judgmental. They can be confused when talking about small defects. For technology, tasks, or wrong decisions, 'Fehler' is usually more natural; for a visible blemish or a damaged reputation, 'Makel' fits better.

Der kleine Kratzer ist der einzige Makel an dem alten Tisch.
The small scratch is the only blemish on the old table.
Wir haben den Fehler in der Maschine vor dem Termin gefunden.
We found the error in the machine before the appointment.