Just like the Square-1 puzzle, you may encounter a parity error near the end. And just like the Square-1 puzzle, I had to refer to Jaap's Puzzle Page to see if there was a quick and easy fix. The answer is: Unlike the Square-1 puzzle, no quick and easy fix exists; as you almost have to start all over again! The Triamid has the Hollywood-style plot-twist ending which is PURE EVIL! In the case of a parity error, you must completely mirror the puzzle.
This is the first step to "mirror" the puzle and define what the new color of each side is suppose to be.
Do the Move:
Rf- Lf- T+
After the two corners are swapped, use Pyraminx like moves (L+, L-, R+, R-, etc...) so that the corners of one face have matching sides. This will help you figure out what new color each side is suppose to be.
You know the drill; invert each and every corner by repeating the next move.
Rotate the entire puzzle so that the corner is at the lower-left (of the front side).
Do the move...
Lf- R+ T+ Lf- T- R-
...to invert the corner.
Repeat this step for each and every corner. Meanwhile, you will not disturb any of the others corners in any way.
Merely use simple Pyraminx-type moves (L+, L-, R+, R-, etc...) to match the sides of all 4 corners.
The corners of your Triamid should now be completely mirror-imaged. Meanwhile, the edge pieces are completely scrambled (again!), so you must re-do the entire edge-phase. You could argue that this is bad solution, but you only have to do a parity fix once; and the toll is the same as solving the lowly Pyraminx twice, which is the easiest and quickest of all Rubik-like puzzles.
SO ONCE AGAIN... Place the Edges