Long, hot soaks in winter are a classic mistake [closed]

Why 'a' is placed after 'are' ? Why 'a' 'is used for the plural 'soaks' ? Why it's 'a classic mistake' not 'classic mistakes' ?

Solution 1:

There is a single mistake mention: (taking) long hot, soaks in winter.

Even though one can take multiple soaks, they all represent a single mistake (repeated)

This is a fairly common construction:

Bee-coloured flies are an example of mimicry

Even though many such flies exist, they just represent one example of mimicry.

In your example, if one would use the plural mistakes, I would expect there to be at least one other mistake:

Long, hot soaks in winter and ice-cold baths in summer are common mistakes.