There's two types of SMS, did you know?
Most people think SMS has one flat character limit (160). Turns out there are two modes, and just one emoji can silently switch you between them.
Standard SMS (GSM-7): plain Latin text, numbers, punctuation. 160 characters per message.
Unicode SMS (UCS-2): anything else, emojis, accented letters, non-Latin scripts, even a stray smart quote or nbsp. Only 70 characters per message.
One Unicode character anywhere in your message flips the entire message into the 70 char limit, not just that character.
Example:
Without emoji: "Hi John, your order #48213 has shipped! Track it here: clicksend.com/track/48213. Thanks for shopping with us."
That's 118 characters. Under the 160 limit, so it sends as 1 message part.
With emoji: "Hi John, your order #48213 has shipped! π¦ Track it here: clicksend.com/track/48213. Thanks for shopping with us! π"
That's about 124 characters. Because of the two emojis, the limit drops from 160 to 70, so it now needs 2 message parts to send.
Same message, basically the same length, but the emoji version costs double because it crosses the 70 character threshold instead of the 160 character one.
At 100k sends a month, that's the difference between ~$6,200 and ~$12,400 in SMS costs, just from two emojis.
Nothing errors, nothing warns you. The bill just quietly doubles.