The Sigma 18-35 super zoom has been used on my EOS 10 and RT for years and there has not been problem.

I recently tried to sell it to a guy. We tested on his EOS 50 and it worked well. After 2 hours he called me back saying it "locked up the camera". I met him again and his EOS 50 shows battery low. All modes (P, Av, Tv, M) failed to work.
I tried on my RT and it happened the same. Changing to new batt didn't help.
I switched to other lens on both bodies and it was fine. Strange.

Upon examing the lens, I noticed the apeture was closed (smallest), while in normal case it should be wide open.
I refunded him.
Sent it to Sigma for a month's service at S$90 and they replaced the part, quoting "burnt motor", with the alleged cause of "wear and tear".

Now, instead of selling it off, I ended up spending more on maintenance.

I'm still being sceptical. I didn't have a good memory of the physical appearance of my lens before selling.
An advice to all: record the series no. before doing any transaction.

Well, given benefit of doubt, it could just be the compatibility problem or simply, "wear & tear".