Sounds like she her manipulative e-mail went as exactly as she planned. To make you feel guilty and to put the blame on you for her error. That way she can say it was your fault and not hers!
Seriously, this is why I get irritated with the "public" for their bad behavior and prefer my dog!
The bride was definitely in the wrong just to assume that you would stilll be available to make her cake. But I can also see how she might be upset if you didn't clearly explain your booking policy at the tasting.
It sounds like maybe you need to be more clear when you conduct your tastings that the date is not booked until you receive a deposit and signed contract and that it is first come first served. That doesn't mean you have to pressure them to sign in any way, just be clear that if someone else gives you a deposit for their date first then unfortunately you won't be available to make their cake.
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