While it is improbable to think one will go through life without ever offending someone, here are some faux-paus to actively avoid and the reasons why one should avoid them.
Don’t every tell a peer that they “look tired”. The imagery evoked by this comment is not flattering. Tired persons have darkened rings about the eyes, unkempt hair and maybe disheveled clothes. They cannot concentrate and are probably grumpy. If you are concerned about a peer just ask if they are OK. While asking if someone looks tired is usually meant to be helpful, it can often be misunderstood as a slight. Likewise, saying someone has lost a ton of weight implies they were fat to begin with, instead just tell your peer the look good without commenting on their previous appearance.
Sometimes slight rephrasings of comments meant in support of a colleague can totally change the interpreted meaning. If someone ends a romantic relationship, don’t ever tell your peer they were to good for them. This may be misconstrued as meaning the peer has poor taste in romantic partners. “Their loss” implies no criticism.
If you do need to criticize someone—hopefully in a helpful way—don’t ever tell a person that they “always” or “never” do something. In the real-world absolutes aren’t really factual. No person always or never does something. Often or frequently or another synonym implies a habit and habits can be changed where as absolutes feel written in stone.
Hopefully these tips will get you thinking about other common sayings and phrasings that might get misinterpreted.