Here is the item again:
Chet decided to get a huge Chinese dragon tattooed across his shoulder
blades once the stabbing needle pierced his
skin, however, Chet quickly changed his mind, opting to pierce an ear instead.
Chet decided to get a huge Chinese dragon tattooed across his shoulder blades is a complete sentence. Once the stabbing needle pierced his skin, however, Chet quickly changed his mind ... is a second complete sentence. The spot where these two sentences meet is blank space, making a fused sentence.
To fix the problem, you could put a period after blades and capitalize the O for once.