Make sure to select words from the BIP-0039 word list randomly, not by selecting lucky numbers or words you more easily remember. Using dice may help as well during the selection process. Use a strong passphrase on top just to be sure (ie. when setting up your wallet you not only enter the seed phrase, but optionally can also add passphrases for multiple accounts in addition to your PIN). Make sure to back up your seed phrase.
Part of the last word value is a "checksum" that is derived from the rest of mnemonic. If you're randomly picking words, it is highly likely that you're going to end up with an invalid checksum... from memory the odds of picking a word that includes a valid checksum are something like 8/2048 (there are usually around 8 words that will have the correct checksum out of the possible 2048).
OPs best option, if they're concerned, is to simply wipe the device and set it up from scratch again as suggested above (it'll generate a new random seed).
Aw geez. Thank you for correcting me! I had a feeling that my memory was off but couldn't quite put my finger on it.