The test are ok, your code is wrong. You're showing there the expected result of one test and the input of the next one. The reasoning is also wrong, you're given bills, so you can't split them.

Python 3.4.3 test bugged:

'NO' should equal 'YES'
[25, 100]

Sell 1st ticket, get 25 (total in hand 25)
Change to give: 0
Sell 2nd ticket, get 25 (total in hand 50)
Change to give: 75 (vs 50 in hand)

There are other examples like this, but this is the simplest. The tests need looking at.

Furthermore, this directly contradicts the second example test given:
tickets([25, 100]) # => NO. Vasya will not have enough money to give change to 100 dollars