Plugging in an old USB stick cause the warnings in the last 2 lines below in dmesg and /var/log/messages :
ugen3.3: <LEXR PLUG DRIVE> at usbus3
umass0: <LEXR PLUG DRIVE> on usbus3
umass0: SCSI over Bulk-Only; quirks = 0xc101
umass0:4:0:-1: Attached to scbus4
da0 at umass-sim0 bus 0 scbus4 target 0 lun 0
da0: <LEXAR DIGITAL FILM /W1.> Removable Direct Access SCSI-2 device
da0: Serial Number 03080701020004ABAAA200000000000
da0: 1.000MB/s transfers
da0: 61MB (125952 512 byte sectors: 64H 32S/T 61C)
g_dev_taste: make_dev_p() failed (gp->name=msdosfs/1161350Z)%0A/, error=22)
g_dev_taste: make_dev_p() failed (gp->name=diskid/DISK-%20%20%20%20%20%20%20%20%20%20%20%20%20%20030807010s1, error=63)
This is harmless for my usage (mounting the content explicitly works). I don't know if it would impair automounting it.
My understanding is that spaces are not valid serial number characters and somehow left-padding it like that was wrong from the device manufacturer, but the way FreeBSD quotes them doesn't seem entirely successful.
The issue has apparently been fixed at some point in release 11 (I don't see it after upgrading to 11.0-PRERELEASE r306368, the extraneous spaces are trimmed off the label) but was still present in 10.3-STABLE r305848.