Once you're back home from your travels (or now, if you're long-term travelling), you have to gather the following materials:
a printout of the result page of this search
a photocopy of your passport ID page, the page with the US visa and the page with the US entry stamp;
photocopies of all passport stamps obtained since leaving the US (and in particular the Cuban entry stamp)
any original tickets that you may have since leaving the US (after making copies for your own record);
A dated bank statement showing all transactions since leaving the US.
Any dated original receipts obtained since leaving the US.
A letter explaining in detail your entire trip, stating the date and place of exit from the US, and asking for the exit to be added to the electronic I94.
Send all these materials by registered mail to:
Coleman Data Solutions
Akron, OH 44306
Attn: NIDPS (I-94)
If the evidence is deemed to be sufficient, the record (found here) will be updated, so check it from time to time. Note that Coleman don't respond to any communication.
If, for any reason, the record is not corrected within two months (check regularly), then gather all the above mentioned evidence (except the passport copy and explanation letter) and bring them on your next trip to the US for presentation at the border, so that the record can be retroactively corrected. However, only do this as a last resort, because if you're unlucky enough to get the wrong officer on re-entry, you may well be refused entry and blacklisted in the immigration records in spite of any evidence you present, although as you hold a visa, you can appeal to an immigration judge if it comes down to it.