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Tim Hortons muffins
At Tim Hortons the muffins contain more fat (on average) than the doughnuts. Do yourself a favour and have a doughnut instead.—MattyEH
Anything off the Starbucks "secret menu"
Former starbucks worker here. Please don't order anything off the "Secret Menu". It doesn't exist.
If you want a snickerdoodle, nuttella, or captain crunch frappuchino (or whatever other overly sugery thing someone has since come out with), know the base drink and the modifications, and order that. If you just say the name, it's up to the barista to come up with what's in the drink, and it may not be what the last barista you ordered from put in there.
Chipotle Quesaritos, not because they're bad but because they take forever
It backs up the line like no other. If it's not busy, you're fine but PLEASE do not order a quesorito during our peak hours. Employees will hate you. The people behind you in line will hate you. Everyone will hate you.
(Note: Chipotle employees apparently can't spell "Quesarito.")