Convenient Timing
In D&D and other roleplaying worlds, there is an unspoken law of conservation of experience.
Player characters kill monsters and gain their experience in some sort of weird exchange, like a quickening from Highlander. The amount of experience in the universe is thus maintained. Balanced. As if on some form of cosmic scales. This balance is ruined when a PC dies and is not resurrected: the monster does not gain experience for the kill and the xp is not shared among the party. The xp is just lost. But nature will not tolerate this, as experience can neither be created nor destroyed. And so a new character suddenly gains that experience, levelling up out of the aether and appearing before the party.
The proper way to do it is to send PCs back to level 1 :P
“All those… XP… will be lost in time, like [small cough] tears… in… rain. Time… to die…”
With classic D&D, because it doubles every level, the amount of XP you need to go from level 7 to level 8 is the same amount of XP a character needs to go from level 1 to level 7! So newbies catch right up, and do it quickly with high level characters on their team. Classic D&D is wise…in ways you people wouldn’t believe…
I thought that in classic DnD, you were limited to going up one level per experience handout session, specifically to avoid the fast level up situation you just described.