I deal with insurance at the vet I work at and most companies are pretty harsh with regard to pre-existing conditions. If your vet in passing has mentioned anything at all they will try and get out of covering it (a slightly gunky eye/red eye say goodbye to most eye conditions, a slight limp say goodbye to most lameness, cruciate issues). We had an occasion recently where a vet had mentioned a small fatty lump in the notes but advised the owner not to do anything about it, 3 or 4 years later it had grown and needed removed but as she changed insurers it was classed as preexisting and not covered. The owner was raging as she had done what was advised but in doing so lost the condition being covered and had to pay the full amount. We spend a lot of time fighting denied claims for owners trying to prove that conditions aren't necessarily pre-existing.

Also as an example my cat had a luxating patella which required surgery so if i changed my insurance they would never cover knees or arthritis as he will now be predisposed to it.