Level Busts
I heard the other day a stasistic that between 70 to 80percent of aircraft operating within UK airspace are flown by crews where english is not their first language. CAP371 - a radiotelephony document introduced to try and improve safety by raising RTF standards, especially wrt level busts, seems only to be applied fully in the UK airspaces.
Only once all aircrews operating into this busy airspace adhere to and understand the importance of using these procedures, will there be progress in limiting level busts.
Combine these two points with very busy RT, pilot fatigue after long flights, and tricky arrivals/departures with step clearances, then no wonder level busts still occur.