As an example, I pull up the clipboard, wanting to wreck some perfectly good RAF planes. Spit8 is 15 perks. Perfect, I like crashing Spits. I select it, and as I get ready to click on NE, I notice I've got the wrong fuel load. I click here and there, setting my fuel load, and in the mean time, 10 more whatever-country-I-am log in. The perk mult changes before I takeoff and the Spit costs 19 perks when I finally hit the runway button. I immediately KS myself trying to steal a squaddie's kill, and bam, there go 19 perks.
WTF IT SAID 15 PERKS!
Well, yeah, it was 15 when I first looked, but it was 19 when I took off. I swore there was something buggy about the perk rides, and after repeated assurances it worked, I finally figured this out.