50, 51, 78, 203, 550, 1275, 2606
Identify the incorrect number in the given number series.

The correct option is 550.

To find the incorrect number in the given number series, let us calculate the differences between consecutive terms:

$51-50=1=1^3$

$78-51=27=3^3$

$203-78=125=5^3$

Notice that the differences between successive terms are the cubes of consecutive odd numbers: 1³, 3³, 5³, 7³, 9³, 11³, ...

Following this pattern, the term after 203 should be obtained by adding 7³ to 203:

$203+7^3=203+343=546$

However, the series provides 550 instead of 546.

We can verify that replacing 550 with 546 maintains the pattern for all subsequent numbers in the series:

$546+9^3=546+729=1275$

$1275+{11}^3=1275+1331=2606$

Since all subsequent terms match perfectly when 550 is replaced by 546, the incorrect number in the series is 550.