Hi guys,
I just was about to Add my funds (not clicked yet!) to my TJ account as my balance was $34 and in 1 second my balance got to -$13. Have you ever see this? My spend was not so big, so I not understand how it could happen. Hopefully it is just a bug?

WTF now I got to -$76 
Check you tracker if you spend this much.
Been bidding first for a huge demo? If so, overspend is very likely. Get on phone with them or Skype, they're usually easy to reach from my experience. Not uncommon to overspend if you're being top bidder in a huge geo, like US etc. basically anything with shitloads of impressions available.
I'm on pretty small demos, before it was just okay.
But I already connected with them and they are experiencing some issue and have a critical ticket for this:
I'm in the same boat as you erikyepes, must be a bug. No increase in traffic but huge increase in negative spend
Seems to be fixed now.
I am also having some weird things. The TJ analytics shows different values from the daily left value...
It's normal with TJ. If you set $20k budgets and the traffic comes in fast ( broad demo ) , it can go negative as much as $300.
Similar thing happened to me. Although, bugs can be a great thing, when you can exploit them 