External air-con machines offer a hideaway for all manner of small creatures. Insects are unlikely to cause serious problems to machines, but it's not uncommon for bugs to block the narrow condensate drainage pipes, which can cause units to drip indoors and/or damage interior walls. Geckos are frequent visitors, seeking safety and warmth. Usually they pose little problem (and eat the insects!) but they can sometimes crawl across a live circuit, not only meeting a sudden end but also causing an electrical failure in the process.
Systems that remain unused for long periods can provide a tempting home - Penguin's technicians have even found units packed with nesting bats! This poses a problem, as the system should remain unused until the young are able to leave the nest.
So, to keep the critters at bay and make sure your systems are in good order, the best advice is to run systems from time-to-time, keep away any rubbish / foliage, keep the outer casings clean – and be sure to book an annual maintenance.