Mocha is returning random failures for passing tests

i'm developing tests for my nodejs project using mocha. I have a main Test.js file with several require statements to include module related tests, i.e:

Test.js:

require('./UserTests.js')
require('./ProductTests.js')
require('./OtherModuleRelatedTests.js')
...

There are one hundred tests. For some reason i get random failures when running them altogether, however this is not happening if i manually add it.only() to the test and run it separately.

I am starting my App just once.

Did this ever happen to any of you?