Be very careful in potentially insecure environments. This includes in particular insecured Wifi connections in hotels or the use of PCs in Internet cafes. In both scenarios there is always the possibility that sensitive information such as login / password will be spied and later abused.
You should also be very cautious in the case of technologies with which you may not be as well familiar or which have not been implemented a sufficient protection by default (smartphones etc.). Missing knowledge can be easily exploited here and the users can be brought to wrong actions (for example, the unconscious installation of malicious software).