This has happened to me twice in the last month...and almost never before that.
I suspect you know the drill:
Get the bill, review the charges, give them my card, the card & unsigned slips return, I write in the tip (and also write it on my copy for reconciliation), and leave...

Then I find on my statement that the charge is higher...tried to think if my handwriting was poor or they typed it in correctly or (more cynically) is this a cute way of getting more in tips?
In each case I wasn't "punitively" under-tipping...and the amounts were small enough so that the time lost in pursuing them (less than $2 each time) was too much.

So does this happen with any frequency to the FT community?