Commercial support: send email. You get an automated message back with a ticket number. Around 6:30 AM you get a call from Delhi that satisfies the 4 hour response SLA, and the support analyst says that they will look into it. At 11 AM you get a request for a dump and certain logs, some of which you had already sent them...
Open source support: after getting zero google results, you look through the source code while running a debugger. It takes you two hours to find the place where it makes an assumption that a signed variable will never be negative, an hour to test your fix, and five minutes to send a patch to the mailing list. It is coming up on 6:30 AM, and you conference in the two people who need to approve emergency changes in order to get that approval. (This is a bank. You can't unilaterally make a production code change.) Everything is deployed before 8 AM.