The Countdown Reflection - S5-E24
Other mistake: Raj doesn't get over his selective mutism until the end of season 6. However, he very prominently speaks during Howard and Bernadette's wedding without alcohol or meds. In the season 6 episode where he finally does speak without any assistance, Penny makes a big deal about it so it's shocking when he does it here with no big shocks.
Chosen answer: It does. The variable cleanliness of the cab and the habit of taxi drivers to engage customers in small talk would be too much for Sheldon to handle, and not an option for him.
Captain Defenestrator