Oh yeah, #PSA if you have fillings from 10+ years ago, it's probably a good idea to get them checked or replaced. Actually, just go see a proper (and probably more expensive) dentist who can give you a full proper check too.
So the dentist was telling me that olde-skool dental practice was not to drill too deep, and apply a gloop of medicinal paste before doing the actual filling. My X-ray showed a big hollow where the gloop used to be and it's probably where the tooth decay and crack started. Modern dentists just drill more off the top to be sure there's no decay and put just filling on top.