When is 2+3 not equal to 5?

Answer 1

If you have two things and then introduce three more things you will always have five things. That is true for every situation. However, if you are working in Base Four you can rewrite 2+3=5 Base 10 into 2+3=11 Base 4, however these two numbers are equal to one another so you still have the same amount of things.

Answer 2

When the measurements are different

Ex

2 miles + 3 kilometers is neither 5 miles, not 5 kilometers