Japanese Language(help)?

Question:

Poketo ni saifu o iremashita ga otoshite shimaimashita.

Can you explain what does the "Shimaishita" represent,I don't understand in what situation or expression will we be using "Shimaimashita".

Answer:

"shimai mashita" sounds "something was done but it's not what the subject wanted"

difference between:
-"otoshi-mashita" (not "otoshi-maishita") and "otoshite shimai-mashita"
"you drop something" (flat meaning) <--> "you drop something (unexpectedly/ by accident)"

-"(saifu o poketto ni) ire-mashita" and "(saifu o poketto ni) ire-te-shimai-mashita"
"you put your wallet in your pocket" (flat meaning) <--> you put your wallet in your pocket by mistake/accident
----- one more
keeki o tabemashita --> "I ate a cake"
keeki o tabete shimai mashita --> "I've eaten a cake (e.g. although I'm on a diet / although I knew it was my little brothers.)"