Various searches to help find missing/incorrect tags:

• source:*www.pixiv.net*
• Incorrect pixiv source. Some of these can't be fixed because the pixiv account has since been deleted.
• source:http:* arttags:0
• Source but no artist. This search includes non-Pixiv posts.
• swimsuit -bikini -one-piece_swimsuit -school_swimsuit -competition_swimsuit -sling_bikini
• Posts tagged swimsuit but not with the specific kind of swimsuit.
• lying -on_side -on_back -on_stomach
• Posts tagged lying but not specifically how the character is lying. on_back isn't very important but on_stomach and on_side should be tagged.
• oral -fellatio -cunnilingus
• 69 -fellatio -cunnilingus
• Posts possibly missing a fellatio or cunnilingus tag.
• group_sex -gangbang -threesome -orgy
• Usually one of the more specific tags is applicable.
• anal -fingering -insertion -tentacles -fisting -sex
• vaginal -fingering -insertion -tentacles -fisting -sex
• Missing sex/insertion/etc tags.
• doggystyle -sex
• group_sex -sex
• rape -sex
• Possibly missing a sex tag.
• ~multiple_penises ~multiple_fellatio ~multiple_paizuri ~double_handjob -group_sex
• Posts that maybe should be tagged group_sex.
• *_(cosplay) -cosplay
• Posts missing a cosplay tag.
• rating:s shota
• rating:s loli
• Posts that probably shouldn't be tagged loli/shota.
• penis -rating:e
• pussy -rating:e
• cum -rating:e
• sex -rating:e
• footjob -rating:e
• fingering -rating:e
• masturbation -rating:e
• oral -rating:e
• paizuri -rating:e
• rape -rating:e
• Probably misrated.
• width:>=3200 -absurdres
• height:>=2400 -absurdres
• width:>=1600 -highres
• height:>=1200 -highres
• width:<=500 height:<=500 -lowres
• filesize:>=10M -huge_filesize
• Missing resolution tags. Flip the signs to find posts that have the tags when they shouldn't.

EDIT: Refined a few searches and added some new ones.