oh-oh, debris!

OWWWWW,
after giving up on my own rebuid, I bought a professionally rebuilt one. I messed up that one too...

I wanted to change the stock cooler line nipples for hose-nipples. Of course one of them broke off. The back one.
Well, I bored it out, paying special attention to keep little particles from falling in the casing.
At the last diameter I used, of course the very shap drill bit bit into the material and moved some 1,5"into the casing. It got out and I cleaned the 1/8 NPT threat with a new tap, vacuumed the daylight out of the channel and put everything together. I put the tranny back under the car and hoped for the best. As I messed with the return line to the transmission I guess any left particles will come into the pan and will stay there, or in the filter.
Special attention to the shift and kickdown adjustment, fluid level, we made a try. I successfully drove out, everything works. But, after a minute, everything is slipping. No forward or backward movement at all. My initial plan was to torch the s.o.b.
We pushed the car back in, restarted it and at first there was movement again! Which quickly ended again. Fluid level seems ok.

Well, before getting my *** kicked by the trans-shop, any guesses what can be the couse? Converter has been rebuild, was tested okay, has been used in another setup, can I have f**d that one up too?