(Screenshot by Cooba, used with permission.)

Odd as it seems that this should happen twice within a small time frame, here's another example of the B's. It looks like an edited screenshot, based on the x position, but Cooba says it's because the rest of the number is missing... which suggests a reason that this happens. (And I guess the hearts are also missing?)
I'm guessing this is basically the same thing as when you host a big level, and people have various letters missing when they join the server, which is later fixed when they rejoin as they're not downloading anymore. (See also: MCE's in multiplayer.) So, when the level is really big (as Cooba says this Raging Inferno level is), or just is somewhat straining (lighting more than 100% has been known to cause problems for JJ2 before), sprites can occasionally go missing or get replaced. Oddly, it was again quotation marks that were replaced by the B's, suggesting that maybe some other characters would create other giant letters?