It seems that when using an active check of type perf_counter(whatever, X) with a large X value, after a host restart, the agent sometimes gets stuck sending trash information.

I have seen this happen 2 times with these counters, monitoring different servers.
perf_counter[\MSExchangeTransport Queues(_total)\Retry Remote Delivery Queue Length, 300] – it would sometimes get stuck sending 17050624
perf_counter[\MSExchangeIS\RPC Averaged Latency, 300] – it would get stuck sending 100

As far as I can tell it happens only when X is larger than 60 (I only use 30, 60 or 300 in my templates) and gets back to normal after an agent restart. It also seems to happen just after a server restart, so it may be that the windows perfcounter API simply does not have enough data to generate an average for 300sec.