I had a questionable experience with a vet last year. Had my male cat booked in to be desexed through one clinic, paid upfront (I do this because a family member often helps by bringing the cat home for me) and went about my business. They phoned an hour or so later saying my cat could not be desexed because of a pre existing health issue, however I would not be refunded the cost of the operation because they had to perform a consultation to tell me he wasn't well enough. The consultation included listening to his heart for a few minutes.
Took my kitty back to a different clinic this year, told them about the previous experience, they agreed to book him in and take a look, when they decided not to operate they refunded the whole amount no questions asked. You really do have to be careful on clinics, some seem very grabby.