brain test

whats wrong with this?
AAA
BBB
CCC
DDD
EEE
FFF
GGG
HHH
III
JJJ
KKK
LLL
MMM
NNN
OOO
PPP
QQQ
RRR
SSS
TTT
UUU
VVV
WWW
XXX
YYY
ZZZ

80% of university students could not find the mistakes. Can you?Don't look at the answer until you know it.
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(the title should be: What's wrong with this?-you need a capital W and an apostrophe s)