This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
There is always a risk that your cat may have a bad reaction to a vaccine, ANY vaccine. It is a good idea to wait in your vet's waiting room for 30 minutes after receiving a vaccination, ANY vaccination, to make sure your cat does not have an adverse reaction, and if it does the vets will be on hand to deal with the situation.
There is also some evidence that vaccinations in general may be the cause of tumors (known as postvaccinal sarcomas or fibrosarcomas). The chance of this happening is estimated to be approximately 1-2 in 10,000, but you should be aware of it nonetheless. This has not been limited to FeLV vaccines, in fact it was originally thought to pertain solely to rabies vaccines, but this is not thought to be the case any more. Since this form of cancer seems to have a high recurrence rate, and little is known about it, if you have *strictly* indoor-only cats, you may want to discuss with your vet if the risk of fibrosarcoma is greater than the risk of being exposed to FeLV if the cat gets out. This is an individual decision that will be different for each household. You should contact your vet immediately if you notice any lumps in the vaccine injection area.