Just put 5 rusty bits into the roundpen at once. It took my bits but only gave me 1 extra training space, but still counted it as 5 bits traded so I couldn't add more.

Something similar has happened to me today

If I accidently use a rusty bit while I still have openings in my round pen, the game will tell me & not take my bit (this is good). When I reach my limit of 5 bits on each of my round pens it tells me & will not take my bit (that is good).

But Today, I accidently clicked on one rusty bit twice in my first pen. The other four bits I didn't. Here is what I ended up with after the game told me I could use no more bits

Same here a few weeks ago. It's like the newest rusty bit negates the others xP