PDF Stumper - scrambled characters (FPDF Library)

At work I have a block of code that creates PDF receipts. On occassion, and seemingly at random, the characters of the PDF come out scrambled like this…

Does anyone have any idea what can cause this to occur? If relevant, the FPDF generator (www.fpdf.org) is the engine being used for this.

This page hints at a possible embedding issue…

Is there a way to test to see if FPDF is fully ebedding the font file?