This understanding was able to mature to the extent of public warnings about it by the 1970s. (sciencedaily.com)
Essentially, if the staff was aware of the danger but didn't warn you about it with a sign or failed to clean up or fix the issue, they may be liable. (himpro.ca)
Luckily they always warn you about it when they set down the tray. (malpaisbeach.com)