Just saw a post on assholedesign about getting charged absurd amounts after free trials end and realized people could benefit from this trick I use.
A lot of sites ask you to put in your CC info when signing up for their free trials, and then they charge you at the end of the trial for the full price. They check if the card is real by charging and refunding $1, or something like that.
Websites like Privacy (I currently don't know any others, but if someone does please link it in the comments) will allow you to create free virtual cards and then cancel it at any time, or set a limit on it. So if you know the full membership is $50/month, you can set a limit for $10, and then the card will decline when they try to charge you if you forget it (or you can simply cancel the card right after you sign up for the trial).
Great way to avoid unnecessary membership charges after free trials.