I have had some problems subculturing mammalian cells in my cell culture room. (I ruled out most other options by sterilizing everything) . To spot the source of the contamination, I left some Horse Serum that I use for the cell media in a petri dish in the incubator over 48h at 37 o C and 5% CO2 - the same conditions as needed for the cells, and I see a lot of growth - it looks like something. I also checked the freshly thawed Horse Serum, and it looked like it had some stuff. Is it common to see funky things like in this Horse Serum image?

(All images are taken at 10x magnification)