#1 Not thinking about what an auditor will want to see
Testing is of course closely related to being able to demonstrate that the systems used by an organisation have been rigorously tested and that evidence of this testing, and the results, can be provided for quality audits such as ISO.
#2 A cheaper but more difficult to use system will cost more in the long term
When considering an automated testing framework, one of the main considerations is ease of use. Cheaper solutions will often compromise on usability and this may seem like a saving in the short term, but will always turn out more expensive in the long term.
#3 Not considering how defect management will be automated
Anyone who has been involved in quality assurance will know that a large proportion of the time spent testing a system, is spent recording defects found during testing rather than executing the tests themselves.
Defect logging and management is a laborious, manual and time-consuming task, especially if it is done to a standard that meet with compliance guidelines. This automatic processing of defects speeds up the test/fix cycle times making the whole testing process faster and more efficient. Selecting a test automation framework without automated defect management will likely not provide the benefits that you may think it will.