Instructions mention nothing about handling numbers differently than letters, but tests seem to expect numbers should be disregarded from trimming. (I was using the c# kata)

Test Failed
  Expected string length 7 but was 6. Strings differ at index 0.
  Expected: "3f39j00", ignoring case
  But was:  "f39j00"

If some tests fail, re-check the worth values whereever you hard-code them into your solution....

I had a typo that drove me nuts and I was convinced the tests were wrong for ages! ;)