Actually, I'm not sure it's possible to have bad Marmite (aka Vegemite). However, the thought that it's possible for somebody to experience hallucinations as a result of a bad Marmite hit amuses me.

In that last panel, Isaac is turning beetroot because Felicia still isn't aware he was the one who sent the anonymous Valentine's card -- which she is alluding to in the third panel.

Panel 1: Felicia is visible on a computer screen, talking to Phillip and Isaac back on Tranquility Base.

Caption:

The next day…

Felicia:

He just started talking to me in the chatroom, calling himself "System", only he told me to call him "Eric".

It's just so weird.

It makes you lot on Tranquility Base look positively normal!

Panel 2:

Isaac:

That is really freaky, please take care!

Phillip:

Fel, you know what this means, don't you?

Panel 3:

Felicia:

That I've attracted another stalker weirdo?

Panel 4: Isaac has gone bright red with embarrassment. Nobody else has noticed.

Isaac (to himself):

!!!

Phillip:

No, it means you ate some bad marmite on your toast.

It's the only logical explanation.