I grow LNCaP prostate cancer cells in T flasks (vented neck, filter cap) in RPMI 1640 medium supplemented with 10% FBS and 1% Pen-Strep. While subculturing the cells, I am observing a strange pattern from past 2 months. When I split one flask into 3, one of the three flasks show contamination (bacterial) after 1-4 days. The other two stay fine. As I subculture them, I find one flask from again contaminated with bacterial infection. This contamination pattern is sporadic. I have changed and checked my medium and buffers multiple times and all seem to be fine. What could be the reason for this random contamination of flasks in between?