There is a group of testers, that adopt certain behaviours, when it comes to testing.
Sometimes, they are happy they found a bug, just so they can stick it to the person, who wrote that part of the program. Sometimes, they try to impose quality by meticulously entering bugs into a defect tracking system. Sometimes, they tend to keep metrics about the number of defects found in a release. Sometimes, they reprimand programmers for supposedly poor-quality work or for not following proper procedures or coding standards. Sometimes, they decline testing builds that don't have adequate documentation. Sometimes, they are convinced that programmers need drilling and are there to happily provide it.
Those people are the quality police.
Don't be a quality police officer.
Testing does not automatically increase the quality of the product. Software testers are people responsible for testing software (du-uh). They need to report what do they see and what is lacking.
If errors, defects or failures are communicated in a constructive way, bad feelings in the team can be avoided.