Using the 20% or 10% EXP Boost Spheres in Combination with the 35% boost from Lvl 50s can lead to crazy amounts of percantages which arent only visual.
By simply equiping 2x 20% Exp Spheres (or any other combination) on all your heros. If you switch those heros you gain 40% EXP boost, but removing the Hero does not reset your boost counter until you switch a map. I've tested this with my heros on Campaign Normal Harbinger and instead of gaining the usual 550k+ EXP from 105% boost, I gained 1,2M+ EXP.
If you "abuse" this bug properly you can level your hero from 1 to 50 with a single map.