This is the 6008th most frequent Russian word.
случайно
"Accidentally" or "by chance."
Here, 'случайно' (accidentally) indicates an unintentional action of forgetting.
Я случайно забыл взять ключи.
I accidentally forgot to take the keys.
In this sentence, 'случайно' is used to politely inquire about the possibility of an occurrence.
Ты случайно не видел мою книгу?
By any chance, have you seen my book?
Here, 'случайно' (by chance) signifies that the meeting happened unexpectedly without any prior arrangement.