[GENERAL] Why does a SELECT query cause "dirtied" buffers?

When using explain (analyze, buffers) things like this can show up:
        Buffers: shared hit=137 read=6545 dirtied=46

Given the description in the manual:
    The number of blocks dirtied indicates the number of previously unmodified blocks that were changed by this query

I am a bit surprised why a SELECT (read-only) query would cause buffers to be dirtied?

Under which situation does a SELECT query change a block?

Regards
Thomas 




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