Most journals require that any research submitted to them must not have been published elsewhere. On the surface, this makes sense, especially in the pre-intern

Most journals require that any research submitted to them must not have been published elsewhere. On the surface, this makes sense, especially in the pre-internet world in which scientific publishing developed. You wouldn't want to allow someone to submit the same paper to multiple journals to inflate their publication count. However, things like preprint servers and blog posts have become a gray area with respect to these rules, and different journals have different policies. I haven't heard of any paper being rejected due to a blog post being considered "prior publication", but I also wouldn't be surprised if I heard of it happening. But rejections due to using preprint servers definitely do happen fairly commonly. The idea of preprint servers is unfortunately still fairly new in the biomedical field and is just starting to gain traction in the past year or two, so many journals still don't even have an explicit policy on preprints, leading to surprise rejections.