The problem was not that the variable was abbreviated. The problem was that the abbreviated variable was so similar to another abbreviated variable that was used for a similar purpose.
I would add - there's a bit of a difference between an abbreviation that just keeps something from being ungodly verbose (20 letters instead of 50), and an abbreviation that shortens something so much that the original meaning is completely lost ("Ttpfe"). This is especially true for things in context - "time of ten percent height on the falling edge" is needlessly verbose, but in context "ten_percent_falling_edge" would probably be perfectly fine. And indeed, that's really just an expanded version of "Ttpfe", which is what was used anyway.
If you shorten everything to five-letter names, it's not really that surprising that it becomes an issue - And I mean, what's the real point of abbreviations when they are so short that it makes it harder to read the code?
Edit: It's worth pointing out though - if this is really old C code it may be justifiable. Back in the olden days of C (Older then C89 at least), only the first 8 characters of a symbol actually mattered. "blahblahone" and "blahblahtwo" would resolve to the same symbol. So shortening in this way could be necessary.