@Gouranga  
We learn from the best.

@Lawful Girly  
Yeah sure, by overthrowing foreign governments and killing millions in the process

@Gouranga  
Not a problem, rest of the world. It’s what we’re here for.

@Lawful Girly  
Oh! thanks ’murika, for explaining it to me, makes more sense now.

@Yet_One_More_Idiot  
As many people have already said, both are correct. It’s just different transliteration systems being used.
 
Unlike Chinese and Korean, Japan does not mandate a specific transliteration scheme for Japanese, so there are two or three major ones in use by different people/sectors. And Japanese themselves can sometimes be creative in how they romanize their own names/products.
 
The ん character in Japanese is usually pronounced “~~n” ~~ and stricter Romanization schemes always label it as such - but its pronunciation is often “-m” before certain consonants like ‘b’ or ‘p’. It can even be pronounced “-ng” sometimes.
 
“Senpai” is correct, but “Sempai” (how I learned it) is how it’s actually pronounced. Make sense?

@atalarikt  
That’s because b, p and m are all bilabial consonants, so mb/mp roll off the tongue more easily than nb/np. Same with d, t and n; and with g, k and ng.

@Yet_One_More_Idiot
 
@Gouranga
 
It’s not a mistake, it’s just a different transliteration system being used.

Sugarcoat looks even cuter with the blush

@atalarikt  
Whoops, spelled, not pronounced. Herp-a-derp.

@Lawful Girly  
Yeah, I’ve seen a lot of people spell it phonetically lately.

@Lawful Girly  
Some other languages, such as Spanish and Catalan, also do that. This is especially common in those two languages, sinec “v” is pronounced as “b”.

@Gouranga  
In Japanese, “n” is pronounced as “m” before certain syllables, most notably those with p or b. Hence when “newspaper”, shinbun, is pronounced as shimbun.

@Yet_One_More_Idiot  
A lot of people seem to make the typo recently.. I have no idea why. I’m personally concerned about it, to say the least..

Sugarcoat: I hope Senpai doesn’t notice me!
 
…also, shouldn’t it be Senpai, not Sempai? I’m sure I’ve only seen it spelt with an N…

Tsundercoat?