I've been seeing these over the last couple of days:

	Mar 29 05:07:12 paradoxa /netbsd: sd0(esp0:1:0):  Check Condition on
	CDB: 0x08 0
	4 d3 84 10 00
	Mar 29 05:07:12 paradoxa /netbsd:     SENSE KEY:  Recovered Error
	Mar 29 05:07:12 paradoxa /netbsd:    INFO FIELD:  316306
	Mar 29 05:07:13 paradoxa /netbsd:      ASC/ASCQ:  ASC 0x18 ASCQ 0x89
	Mar 29 05:07:13 paradoxa /netbsd: 

	The CDB fields vary and the INFO FIELD varies, but it looks like the
	ASC/ASCQ field is the same from time to time.

Time to buy a new disk.

Don't know what the 0x89 is but 0x18 is some type of a recovered data 
error.  If you're getting lots of them it probably means your media
is probably going bad.  It will probably get worse until you start losing
data.  I recommend backing everything up and replacing that disk.

Eduardo