I had just thawed an old batch of BCS (from 2018), which had previously been thawed (probably due to freezer malfunction) once in 2020. The BCS stock appeared cloudy upon thawing, which I initially thought was normal for serum. I then added the serum to my media for cell culture work.

After 24 hours, the complete media remained red. However, I noticed tiny dots or rod-like structures in the media (I'm not entirely sure of the shape). (Images attached at 10x magnification).

Only afterward did I observe the BCS under the microscope and saw many floating structures, though I’m still unsure what they are.