What are the mistakes commonly made in cell culture experiments?

Some of the more common mistakes made in cell culture experiments include: 

• Keeping cell lines in culture continuously instead of returning to frozen stock. 
• Allowing cell cultures to become fully confluent. Consult the ECACC cell culture data sheet right for the best option. Alternatively, sub-culture at 70% to 89% confluency. 
• Allowing the media to go past the expiry date. Once serum and glutamine are added, the media shelf life is between 4 and 6 weeks at 4 °C. 
• Using antibiotics continually in the culture medium. This could mask underlying contamination by creating an appearance of antibiotic resistant strains. 
• Handling cells from unauthenticated sources in the primary cell culture suite. As a best practice, only use cell suppliers that authenticate cells for seed stock. 
• Allowing waste to accumulate in the incubators or within the microbiological safety cabinet. 
• Not cleaning water baths regularly. 
• Having too many people in the lab at any one time.