Managing a small team - poorly.
I was in charge of testing a legacy calculations engine together with two scientists, for whom I set up a python and interop environment so they could test the engine easily.

The two were very excited at the thought of validating the calculations and in fact found many bugs.

I was very supportive, told them to fix the bugs and gave them a pet on the back.

All three of us were happy the legacy engine is shaping up, that's until my boss heard of it, and boy did he grill me hard for it.

Turns out our efforts were highly unappreciated by the client, whose only request was that we test the engine and report the bugs. Not to fix them. My goodwill cost the company a lot of money, since the client paid by the hour, and was now due a refund. Crap.

It took me a year to finally understood the moral of the story. Which is to always respect the client's wishes and convey maximum transparency to him.